NATURE AND NURTURE 59 



is one thing, while the character of the action itself is quite another 

 thing, this fact tends from its very familiarity, to slip out of the 

 minds of students ; and two views of the nature of the process of 

 development of the living thing out of the germ, which have been 

 argued for centuries, illustrate this tendency. One school of embry- 

 ologists has long held that the egg or germ produces the living thing 

 in virtue of its inherent potency, or specific constitution, which is, 

 in some way, an embodiment of all that is to be unfolded out of it ; 

 while the other school finds, in the stimulus which is given by 

 nurture, in the influence of the external world, and in that which the 

 parts of the segmenting egg and those of the growing organism 

 exert on each other, the explanation of each successive step in the 

 process of development. 



Advocates of these two views have regarded themselves as 

 opponents, but except that latent potency is hard to lay hold of, 

 while mechanical conditions readily lend themselves to experiment, 

 I cannot see why there should be any real antagonism; for the evi- 

 dence that each may be true seems ample. Every change that 

 takes place in the living being, from the beginning to the end of 

 individual life, may be called forth by some mechanical stimulus, 

 either within the body or without ; and yet the outcome of the whole 

 process may be no more than exhaustive knowledge of the nature of 

 the germ would lead one to expect. 



The gun does not go off until the cap is exploded, but it hits the 

 mark because it is aimed. While the distinction between the stim- 

 ulus to a vital change and the nature of the change itself is obvious 

 enough in simple cases, we may easily become confused and lose 

 sight of it in handling complicated problems. 



A hen's egg will not develop without heat and fresh air, and 

 when these are properly supplied it becomes a chick, although 

 belief that the heat causes the chick is too grotesque for the sane 

 mind; for the production of a duckling from a duck's egg in the 

 same nest proves, if any proof be needed, that while the egg will 

 not develop without incubation, the outcome of the process of 

 incubation is the result of the inherent capacity of the egg itself. 



The most notable peculiarity of this inherent tendency or 

 specific constitution of living things is its fitness. The egg not only 

 gives rise to a specific organism, but to one that is beautifully and 



