186 THE PHILOSOPHY OF BIOLOGY 



generations of Paramcecium, for instance, which were 

 bred by Woodruff) which can be produced by ex- 

 perimental breeding. Some species of animals — the 

 Brachiopod Lingula, for instance — have persisted 

 unchanged since Palaeozoic times. Throughout the 

 incredibly numerous generations represented by this 

 animal series, the specific organisation must have been 

 transmitted in an almost absolutely unchanged con- 

 dition. The germ-plasm is therefore continuous from 

 generation to generation, and it possesses an exceed- 

 ingly great degree of constancy of character. This 

 conception of the continuity and stability of the specific 

 organisation is the feature of value in Weismannism, 

 and all that we know of the phenomena of heredity 

 confirms it. But it is pure speculation to regard 

 the organisation as an aggregate of chemically dis- 

 tinct substances, or if we say that this speculation 

 is rather a working hypothesis, then it must justify 

 itself by leading us back again to the results of 

 experience. 



It is, however, not quite accurate to say that the 

 organisation persists unchanged from generation to 

 generation. The offspring is similar to the parent — 

 that is, the organisation has been transmitted un- 

 changed. But the offspring also differs just a little 

 from the parent — that is to say, the organisation is 

 modified by each transmission. In these two state- 

 ments we formulate in the simplest manner the law 

 of organic variability. Organisms may obviously be 

 arranged in categories in such a way that the 

 individuals in any one category resemble each other 

 more closely than they resemble the individuals belong- 

 ing to another category. We may, by experimental 

 breeding, produce an assemblage of organisms all of 

 which have had a common ancestor, or a pair of 



