EDWARD JENNER (1749-1823)

Portrait of EDWARD  JENNER


1798 – England


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1859 – England

‘All present day species have evolved from simpler forms of life through a process of natural selection’

Portrait of Charles Darwin ©

Organisms have changed over time and the ones living today are different from the ones that lived in the past. Furthermore, many organisms that once lived are now extinct.

The orthodox view was that of the Creationists. According to the Book of Genesis in the Bible, ‘God created every living creature that moves….’. Against this background, thinkers such as French naturalist Jean-Baptist Lamarck developed a picture of how species evolved from single-celled organisms.

Darwin’s breakthrough was to work out what evolution is and how it happens. His insight was to focus on individuals, not species and to show how individuals evolve by natural selection. The mechanism explained how all species evolved to become well suited to their environment. Later commentators have characterized this idea as ‘survival of the fittest,’ but this was never a phrase that Darwin himself used.

Darwin was influenced by CHARLES LYELL’s newly published book ‘Principles of Geology’, showing how landscapes had evolved gradually through long cycles of erosion and upheaval and by ‘An Essay on the Principle of Population’ written in 1798 by THOMAS MALTHUS.

The publication of Darwin’s book ‘On the Origin of Species by Means of Natural Selection’ in 1859 generated social and political debate that continues to this day. Darwin did not discuss the evolution of humans in this book.
In ‘The Descent of Man’, published in 1871, he presented his explanation of how his theory of evolution applied to the idea that humans evolved from apes. In modern form the theory contains the following ideas:

  • members of a species vary in form and behaviour and some of this variation has an inherited basis

  • every species produces far more offspring than the environment can support

  • some individuals are better adapted for survival in a given environment than others

this means that there are variations within each population gene pool and individuals with most favourable variations stand a better chance of survival – the survival of the fittest.

  • the favourable characteristics show up among more individuals of the next generation

there is thus a ‘natural selection’ for those individuals whose variations make them better adapted for survival and reproduction.

  • the natural selection of strains of organisms favours the evolution of new species, through better adaptation to their environment, as a consequence of genetic change or mutation.

Knowledge of DNA has enriched the theory of evolution. The modern view is still based on the Darwinian foundation; evolution through natural selection is opportunistic and it takes place steadily.

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GREGOR MENDEL (1822- 84)

1865 – Austria

  • ‘Law of Segregation: In sexually reproducing organisms, two units of heredity control each trait. Only one of such units can be represented in a single sexually reproductive cell’

  • ‘Law of Independent Assortment: Each of a pair of contrasted traits may be combined with either of another pair’

These laws laid the foundation for the science of genetics.

The biologist Lamarck (1744-1829) had proposed a theory of inheritance of acquired characteristics and had suggested that inherited characteristics are influenced by environment. Mendel planted an atypical variety of an oriental plant next to a typical variety – the offspring retained the essential traits of their parents, which meant that the characteristics that were inherited were not influenced by the environment. This simple test led Mendel to embark on the path that would lead to the discovery of the laws of heredity.

Mendel’s aim was to discover ” a generally applicable law of the formation and development of hybrids “. He addressed this by studying the effect of cross-breeding on seven pairs of contrasting characteristics of Pisum sativum, a strain of pea.
His work on peas indicated that features of the plant; seed shape, seed colour, pod shape, pod colour, flower colour, flower position and stem length; were passed on from one generation to the next by some physical element. He realised that each characteristic of a plant was inherited independently, and that the ratios of plants exhibiting each trait could be statistically predicted.

photograph of GREGOR MENDEL ©


A common assumption in Mendel’s time was that when two alternative features were combined, an average of these features would occur. For example, a tall plant and a short one would result in medium height offspring. For seven years Mendel kept an exact record of the inherited characteristics of 28,000 pea plants, taking great pains to avoid accidental cross-fertilization; then he applied mathematics to the results. These quantitative data allowed him to see statistical patterns and ratios that had eluded his predecessors.

From his analysis he found that certain characteristics of plants are due to factors passed intact from generation to generation.
Mendel observed that individual plants of the first generation of hybrids (crossbred plants) usually showed the traits of only one parent. The crossing of yellow seeded plants with green seeded ones gave rise to yellow seeds; the crossing of tall stemmed ones with short-stemmed varieties gave rise to tall-stemmed plants.

The factors determining a trait are passed on to the offspring during reproduction.

Mendel worked out that the factors for each trait are grouped together in pairs and that the offspring receives one part of a pair from each parent.

Contrary to the popular belief of the time, these factors do not merge. Any individual pea always exhibits one trait or the other, never a mixture of the two possible expressions of the trait; only one trait from each pair of factors donated by the parents would be expressed in the offspring, although there are four possible combinations of factors.
This is now described as Mendel’s law of segregation.
An offspring inherits from its parents either one trait or the other, but not both.

He decided that some factors were ‘dominant’ and some were ‘recessive’ and was able to conclude that certain expressed traits, such as yellow seeds or tall stems, were the dominant ones and that other traits, such as shortness of stem and green seeds, were recessive. It appeared that the dominant factors consumed or destroyed the recessive factors – but this could not be the case, as the second generation of hybrids exhibited both the dominant and recessive traits of their ‘grandparents’. Across a series of generations of descendants, plants did not average out to a medium, but instead inherited the original features (for example, either tallness or shortness) in consistent proportions, a ratio of 3:1, according to the dominant factor.
The 3:1 ratio would apply because the dominant factor would feature whenever it was present.

He also noted that the different pairs of factors making up the characteristics of the pea plant ( such as the pair causing flower colour, the pair causing seed shape and so on ), when crossed, occurred in all possible mathematical combinations. This convinced him that the elements regulating the different features acted independently of each other, so the inheritance of one particular colour of flower was not influenced, for example, by the inheritance of pea shape.
This is now described as Mendel’s law of independent assortment.

He first articulated his results in 1865 and in 1866, which was shortly after Darwin’s ‘Origin of Species’ appeared, published them in an article ‘Versuche über Pflanzen-Hybriden’ (Experiments with plant hybrids).

No one before him had attempted to use mathematics and statistics as a means of understanding and predicting biological processes and during his lifetime and for some time after, his results were largely ignored.

Around the time of Mendel’s death, scientists using ever improving optics to study the minute architecture of cells coined the term ‘chromosome’ to describe the long, stringy bodies in the cell nucleus.

The seven traits studied in peas
Type of seed surface smooth wrinkled
Colour of seed albumen yellow green
Colour of seed coat grey white
Form of ripe pod inflated constricted
Colour of unripe pod green yellow
Position of flowers on stem axial terminal
Length of stem tall short

‘There is doubt as to the probity of this Jesuit scholar, some claiming that his data was falsified whilst others argue that it is accurate’
Pilgrim, I. (1984) The Too-Good-to-be-True Paradox and Gregor Mendel. Journal of Heredity,#75, pp 501-2. Cited in Brake,M.L. & Hook, N. Different Engines – How science drives fiction and fiction drives science

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LOUIS PASTEUR (1822- 95)

1865 – France

‘Many human diseases have their origin in micro-organisms’

1862 – ‘Memoire sur les corpuscles organises qui existent dans l’atmosphere’ (Note on Organized Corpuscles that exist in the Atmosphere) – Puts an end to centuries of debate on the theory of spontaneous generation.

Although a chemist, Pasteur is best remembered for his contributions to medicine. His name is used to describe the process of ‘pasteurisation’.
Pasteur proved that living microorganisms cause fermentation. Previously scientists had assumed that fermentation was a chemical process.
Pasteur showed that the alcohol in fermentation was made by the yeast microbe. He also realised that when fermentation went wrong it was due to other germs.

Portrait of Louis Pasteur in his laboratory


In 1863 he showed that brief, moderate heating of wine and beer kills germs, thereby sterilizing the foodstuffs and ending the fermentation process. The process now known as pasteurisation is still used in the food industry.

His investigations led him to believe that microorganisms could also cause disease in humans. Pasteur realized the dangers of infection, but the English surgeon JOSEPH LISTER (1827-1912) is credited with developing and systematizing the notion of antiseptic surgery so that operations could be made safer if an ‘antiseptic’ procedure was introduced to destroy microbes and curb the infections that followed wounds or surgery.

In 1876, Pasteur confirmed the findings of ROBERT KOCH’s discovery of the anthrax bacillus. After EDWARD JENNER’s breakthrough in the development of vaccination against smallpox, little had been done to take advantage of the potential of this treatment against other disease.

In 1882 Pasteur successfully applied his discovery of vaccination by attenuated culture of microorganisms to anthrax and in 1885 to the treatment of rabies in humans.

On 14 November 1888 the Pasteur Institute opened in Paris.




ROBERT KOCH (1843-1910)

1876 – Germany

‘Koch’s postulates – four conditions that need to be satisfied to be sure that a particular type of bacteria causes disease’

Koch developed methods of staining bacteria that enabled him not only to see them under a microscope, but also to differentiate between the various strains of microorganisms that he found.

Koch proved that specific organisms cause specific diseases and in addition, that pollution could spread disease.
He developed methods for obtaining pure cultures of bacteria and laid down Koch’s Postulates.

photo portrait of ROBERT KOCH who devised 'KOCH's POSTULATES' ©


His colleague RICHARD JULIUS PETRI (1852-1921) designed a shallow flat dish that allowed him to grow microorganisms on a solid flat surface, and thus easily separate colonies of bacteria. Until then scientists had grown bacteria in flasks, or injected them into animals.

Koch’s rules for identifying harmful bacteria

  • – the micro-organism must be identified and seen in all animals that suffer the same disease
  • – it must be cultured through several generations
  • – these later generations of bacteria must be capable of causing the disease
  • – the same agent must be found in a newly infected animal as was found in the original victim

Using this set of criteria he identified the organisms responsible for more than twenty diseases, including tuberculosis, salmonella, cholera, pneumonia and meningitis.

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IVAN PAVLOV (1849 – 1936)

1903 – Russia

‘A conditioned reflex is a learnt response to an environmental stimulus’

The process of learning to connect a stimulus to a reflex is called conditioning.

An innate or built-in reflex is something we do automatically without thinking (such as moving our hand away from a flame).

photograph of ivan pavlov and his staff c1925 pavlov-whose-views-formed-the-foundation-of-behaviorism-believed-that-learning-consisted-of-a-series-of-conditioned-responses

Ivan Pavlov and his staff

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