PRINCIPLES AND METHODS OF PALAEONTOLOGY. 379 



mise, a resultant of all the forces which act upon it; and though, like 

 a planet, it tends with an immense force to move in a coarse of its own, 

 yet, like that ])lanet, it is affected and j)erturbed more or less by all sur- 

 rounding conditions. 



Hence, inasmuch as no two living beings can ever possibly have been 

 subjected to precisely the same conditions, it is not wonderful that no 

 two ever were, or ever will be, precisely alike; nor is it strange that 

 species vary in i^roportion to the variety of the conditions to which they 

 are exposed. 



It is needless to do more than refer to fiict^ which lie within every 

 one's experience. No person is unaware of tlie dili'erence in the result 

 produced when two seeds from the same plant, or two animals from the 

 same brood, are exposed to widely different conditions in respect of light, 

 warmth, and nourishment. 



In all such cases, however, the modification is limited in amount, and 

 no modification of conditions will so mask the characters of the species 

 as to prevent their recognition in either the stunted or tiie overgrown 

 individual. For every individual, therefore, it can hardly be doubted 

 that specific characters are licrmanent and immutable. Do what you 

 will with a sheep dog puppy, you will not turn him into a wolf. 



It is obvious, therefore, that thus far the influence of conditions can 

 be shown to have no appreciable effect in permanently nu)difying spe- 

 cies; for, if the offspring of the modified individual were in all respects 

 like its parent before the modification of the latter, it is clear that the 

 whole influence of the modifying conditions would only bring 'c co the 

 same point as the parent ; that the modification in any numlrcr o. gene- 

 rations would go no further ; and that when the influence A these con- 

 ditions was removed, the species would at once return to its primitive 

 and typical form. Thus, suppose a pair of sheep-dog ;^'appies could be 

 converted into greyhounds by a peculiar course of f^od and training ; 

 for anything which has been yet stated they would produce puppies 

 which woukl only become greyhounds under a like course, and if left 

 to themselves, would resume tlieir pure and unchanged sheep-dog char- 

 acter. 



Now, in nature this is not the case, by reason of the great fact of 

 hereditary transmission. Every living being is, it has been said above, 

 the resultant of all the forces which act upon it; the statement is in- 

 complete unless we add : and which have acted upon its parents 



The forces in question are divisible into two classes : the one more 

 powerful, intrinsic, impressed upon the germ, and causing that germ 

 invariably to tend toward the production of a giveii form ; the other 

 weaker, extrinsic, consisting of all those assisting, modifying, or even 

 destructive influences which reside in the surrounding universe, and 

 which are called external conditions. 



For every individual living thing, this distinction into intrinsic and 

 extrinsic forces is absolute; but the law of hereditarv' transmission 

 obliges us to admit that it may not be so for a series of generations. 

 For liereditary transmission means simply, that a modification under- 

 gone by a parent more or less affects its otfspring — the offspring tending 

 to reproduce that modification. Thus in the imaginary instance given 

 above, the offspring of the modified sheep-dog, even if placed in entirely 

 indiflerent conditions, would have a tendency to assume greyhound char- 

 acters. The intrinsic force of that germ, its tendencies, would be thus 

 far modified by the influence exerted by external conditions on its parent. 

 The operation of an extrinsic force on one generation may become in the 

 next an intrinsic force. 



