Heredity / by J.A.S. Watson.

  • James Anderson Scott Watson
Date:
[1912]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Heredity / by J.A.S. Watson. Source: Wellcome Collection.

    32/104 page 30
    30 HEREDITY that actual changes in the nature of the germ plasm are comparatively rare. They are the exception and not the rule in heredity. It is impossible in our smaU space to discuss all the reasons on which this idea is based, but some of them wiU emerge in the sequel. This conception of variation is very different from that formed by Darwin, who studied the subject very thoroughly. Darwin believed that inborn variation was the rule ; that the characters of a species were in a state of flux, so that new variations in one direction or the other could be produced merely by selecting in those directions. One species was formed from another by the accumulation of minute indefinite tendencies in one direction. Darwin emphasised the slowness of the pro¬ cess of change in living things ; Natura non fecit saltum ( Nature does not leap ) was a favourite aphorism of his. The process of change is no doubt slow in the mass : but we have lately come to the opinion that nature does leap—^in fact, that it is only by leaps that change can occur. The slowness of evolution, then, is due to the fact that the leaps are seldom and small. It is perhaps necessary to say that mutations may be either progressive or retrogressive. The change may be a gain of a new character, but is perhaps oftener a loss of a character which had been gained previously in the history of the race. Any variation in which a characteristic of a remote ancestor recurs is termed a reversion. Certain rever¬ sions are due to special cases of recombination, which will be discussed later. But some reversions at least are retrogressive mutations. As regards the causes of mutations it must be ad¬ mitted that we are still in almost complete ignorance. We have had enough of speculation on the matter, but the cases in which we can point to any definite cause are extremely few. Even when a connection has been observed between a certain condition and the occurrence of a mutation, it has been impossible to trace the chain of causes connecting the two. This is probably due to
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