76 the president's address. 



some special purpose to produce one of these results only, but 

 they are inseparably connected and you cannot have one without 

 the other. You cannot have the result aimed at without having 

 also the by-product. 



Now suppose some change in the constitution of the germ- 

 plasm of an organism to give rise to two modifications in the 

 developing soma or body. We may call the change or modi- 

 fication in the germ-plasm GA and the modifications in the soma 

 SA and Sa. SA and Sa will be inseparably correlated with one 

 another through GA, though as for example in the case of 

 white tom-cats with blue eyes, which are said to be generally 

 deaf the connection between them may appear to be quite 

 arbitrary. 



Suppose further that SA proves to be a useful character and 

 Sa a useless one. Then, under the influence of natural selection, 

 SA will be preserved and may ultimately develop into a very 

 perfect adaptation ; but, if so, GA must also undergo further 

 modification, and this modification will likewise affect Set, which will 

 therefore keep pace, so to speak, with SA. Thus a non-adaptive 

 character (S) may undergo progressive evolution, which, though 

 in reality indirectly controlled by the action of natural selection, 

 may appear to be guided by some mysterious vital force or 

 entelechy. 



Now suppose further that as a result of some change in 

 the conditions of life, or merely as the result of its progressive 

 evolution in some particular direction, So- in turn acquires some 

 value in the struggle for existence. Natural selection will, in 

 future, favour its further development directly, and what was 

 at first a mere by-product becomes an adaptive character. Thus 

 adaptive characters may perhaps become linked together in 

 groups, the existence of each group being dependent on some 

 particular property of the germ-plasm through which all the 

 members of the group are connected. 



At the same time non-aclaptive characters may persist side 

 by side with adaptive ones, and even harmful variations may 

 persist if their injurious effects are counterbalanced by useful 

 characters with which they happen to be correlated and which 

 cannot exist without them. Inasmuch, however, as two useful 

 characters are more valuable than one, natural selection will 

 tend to favour the correlation or linking together of adaptive 



