CHAPTER XIII

EVOLUTION

DARWIN'S work has been compared to that of Copernicus and Galileo inasmuch as all these men freed the mind from the incubus of Aristotelian philosophy which, with the efficient co-operation of the church and the predatory system of economics, caused the stagnation, squalor, immorality, and misery of the Middle Ages. Copernicus and Galileo were the first to deliver the intellect from the idea of a universe created for the purpose of man; and Darwin rendered a similar service by his insistence that accidental and not purposeful variations gave rise to the variety of organisms. In this struggle for intellectual freedom the names of Huxley and Haeckel must be gratefully remembered, since without them Darwin's idea would not have conquered humanity.

Darwin assumed that the small fluctuating variations could accumulate to larger variations and thus cause new forms to originate.

It was the merit of de Vries' to have pointed out that fluctuating variations are not hereditary and hence could not have played the rôle assigned to them by Darwin, while discontinuous variations as they appear in the so-called "sports" or mutations are inherited. This was an important step in the history of the theory of evolution. It did not touch the foundation of Darwin's work, namely the substitution of the idea of an accidental evolution for that of a purposeful creation; it only modified the conception of the possible mechanism of evolution. According to de Vries, there are special species or groups of species which are in a state of mutation. He considers the evening primrose on which he made his observations as one of these forms. Morgan and his pupils have observed over 130 mutations in a fly Drosophila. From our present limited knowledge we must admit the possibility that the tendency toward the production of mutants is not equally strong in different forms. Whether this part of de Vries's idea is or is not correct there can be no doubt that variations occur which consist in the loss and apparently, though in rarer cases, in the gain or a modification of a Mendelian factor. If we wish to visualize the basis of such a change we may do so by imagining well-defined chemical constituents in one or more of the chromomeres undergoing a chemical change.

'de Vries, H., The Mutation Theory, translated by Farmer, J. B., and Darbishire, A. D., Chicago, 1909. Species and Varieties. Chicago, 1906. Gruppenweise Artbildung. Berlin, 1913.

This way of looking at the origin of variation has had the effect of putting an end to the vague speculations concerning the evolution of one form from another. We demand today the experimental test when such a statement is made and as a consequence the amount of mere speculation in this field has diminished considerably.

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It is possible that any further progress concerning evolution must come by experimental attempts to bring about at will definite mutations. Such attempts have been reported but they are not all beyond the possibility of error.1 The most remarkable among them are those by Tower who by a very complicated combination of effects of temperature and moisture claims to have produced definite mutations in the potato beetle. The conditions for these experiments are so expensive and complicated that a repetition by other investigators has not yet been possible.

It is, however, still uncertain whether the mere addition or loss of Mendelian characters can lead to the origin of new species. Species specificity is determined by specific proteins (Chapter III.), while some Mendelian characters at least seem to be determined by hormones or substances which need neither be proteins nor specific for the species.

For a critical discussion of the details, see Bateson, W., Problems of Genetics, New Haven, 1913, Chapter X.

CHAPTER XIV

DEATH AND DISSOLUTION OF THE ORGANISM

1. It is an old saying that we cannot understand life unless we understand death. The dead body, if its temperature is not too low and if it contains enough water, undergoes rapid disintegration. It was natural to argue that life is that which resists this tendency to disintegration. The older observers thought that the forces of nature determined the decay, while the vital force resisted it. This idea found its tersest expression in the definition of Bichat, that "life is the sum total of the forces which resist death." Science is not the field of definitions, but of prediction and control. The problem is: first, how does it happen that as soon as respiration has ceased only for a few minutes the human body is dead, that is to say, will commence to undergo disintegration, and second, what protects the body against this decay while the respiration goes on, although temperature and moisture are such as to favour decay?

The earlier biologists had already raised the question

why it was that the stomach and intestine did not digest themselves. The hydrochloric acid and the pepsin in the stomach and the trypsin in the intestine digest proteins taken in in the form of food; why do they not digest the proteins of the cells of the stomach and the intestine? They will promptly digest the stomach as soon as the individual is dead, but not during life. A self-digestion may also be caused if the arteries of the stomach are ligatured. Claude Bernard and others suggested that the layer of mucus protected the cells of the stomach and of the intestine from the digestive enzymes; or that the epithelial layer had a protective effect. Pavy suggested that the alkali of the blood had a protective action. All these theories became untenable when Fermi showed that all kinds of living organisms, protozoans, worms, arthropods, are not digested in solutions of trypsin as long as they are alive, while they are promptly digested in the same solution when dead. This is in harmony with the fact that many parasites live in the intestine without being digested as long as they are alive. Fermi concluded that the living cell cannot be attacked by the digestive ferments, while with death a change occurs by which they can be attacked. But what is this change? Fermi seems to be inclined to think that the "living molecule" of protein is not hydrolysable (perhaps because the enzyme cannot attach itself to it?),

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Fermi, C., Centralbl. f. Bacteriologie, Abt. 1, 1910, lvi., 55.