THE ANIMAL IN THE WORLD 545 



to the structure of the crystalline variety from which it was 

 formed. 1 True the changes are utterly unlike in the two 

 cases, but it seems just possible that the same principle 

 may underlie both of them. It must be confessed, how- 

 ever, that all such comparisons are on the face of them 

 extraordinarily far-fetched, and they do not touch the fact 

 that the process of development is not a simple change, 

 but involves an immense number of co-ordinated changes 

 directed to a common end. (iii) The feature of heredity 

 which is certainly peculiar is that it enables the germs of 

 different kinds of living beings, while they are essentially 

 alike, to behave differently in circumstances which are 

 essentially the same — that is to say, to increase their un- 

 likeness out of all proportion to the unlikeness of their 

 surroundings. It is a remarkable fact that not only are 

 the germs of organisms always more alike than their adults, 

 but in some cases the closest microscopical and chemical 

 investigation will reveal only the smallest differences 

 between the germs of animals whose adults are extremely 

 unlike. And in most of these cases the conditions (of 

 warmth, nourishment, etc.) in which the germs develop 

 differ as little as the germs themselves. The most striking 

 case of this close resemblance of the germs and their 

 conditions of development in very different animals is to be 

 found in mammals. It is not at present possible to con- 

 ceive how the great differences between a man and a horse 

 can be due to the small differences which can as yet be 

 recognised between their ova. All this, of course, is not 

 to deny that there is anything in the germs in question 

 which brings about the differences in their adults. It is 

 merely to assert that the development of the living germ is 

 directed in a way which is as yet incomprehensible and 

 cannot be paralleled in lifeless matter. In heredity, then, 

 we find that reproduction does present a property peculiar 

 to living beings. 



6. It will be seen, however, that this property is of a 

 directive nature. That is to say, it is a manifestation of 

 that purposiveness which we have already seen to govern 

 all the activities of living beings, and to be peculiar to 

 them. 



1 By melting and pouring into water. 



