CHAP. XX Heredity 323 



But every animal is usually a little different from its 

 parents, and except in cases of " identical twins " cannot be 

 mistaken for one of its fellow-offspring. The proverbial 

 " two peas " may be very unlike. Organisms are variable, 

 and this is natural, for life begins in the intimate 

 mingling of two units of living matter perhaps very dif- 

 ferent and certainly very complex. The relation between 

 successive generations is such that the offspring is like 

 its parents, but various causes producing change diminish 

 this likeness, so that we no longer say " like begets like," 

 but " like tends to beget like." 



There are, I think, two other important facts in regard 

 to heredity, but both require discussion — the one because 

 some of the most authoritative naturalists deny it, the other 

 because it is difficult to understand. 



I believe that some characters acquired by the parent as 

 the result of what it does, and as impacts from the surround- 

 ing conditions of life, are transmissible to the offspring. In 

 other words, some functional and environmental variations 

 in the body of the parents may be handed on to the 

 offspring. This is denied by Weismann and many others. 



The other fact, which has been elucidated by Galton, 

 is that through successive generations there is a tendency 

 to sustain the average of the species, by the continual 

 approximation of exceptional forms towards a mean. 



2. Theories of Heredity — Mstorical retrospect. — 

 Theories of heredity, like those about many other facts, 

 have been formulated at different times in different kinds 

 of intellectual language — theological, metaphysical, and 

 scientific — and the words are often more at variance than 

 the ideas. 



(a) Theological Theories. — It was an old idea, that the 

 germ of a new human life was possessed by a spirit, some- 

 times of second-hand origin, having previously belonged to 

 some ancestor or animal. So far as this idea persists in the 

 minds of civilised men, it is so much purified and sublimed 

 that if the student of science does not believe it true, 

 he cannot wisely call it false. 



[p) " Metaphysical Theories!' — For a time it was com- 



