xv THE PHILOSOPHY OF ZOOLOGY 



the cells of the adult body must, therefore, in these cases retain 

 in a certain degree the faculty of reproduction, and must contain 

 germinal substance. The germinal substance concerned in re- 

 generation must, it is of importance to note, be capable of being 

 stimulated into activity in a certain definite direction by an influence 

 brought to bear upon it from without. 



In the Vertebrata the power of regeneration, if we leave out 

 of account the various epidermal structures, is exceptional ; and 

 where it occurs (most Amphibia, some Reptiles) it is confined 

 to the limbs, jaws, lens of the eye, or the tail. In the highest 

 Vertebrates there is no power of regenerating such parts when lost, 

 and the capacity for reproduction is confined to the sexual cells. 



A remarkable persistency characterises these reproductive cells. 

 By their means there are handed down from one generation to 

 another, with little alteration, all the characteristics of the species 

 of plant or animal. This special faculty of the reproductive cells 

 is the faculty of heredity. 



Heredity does not imply absolute fixedness of all the character- 

 istics inherited by one generation from its predecessor. On the 

 contrary, as already pointed out, variations are constantly pre- 

 senting themselves. Some of the variations which animals exhibit 

 are a direct result of the action of surrounding conditions, or of 

 the use or disuse of parts, on the fully developed animal ; we can 

 in some cases actually cause the animal to change to a more or 

 less marked extent by placing it under different conditions. 

 Another set of variations produced by the action of external 

 influences on the organism only appears if the action takes place 

 in the course of development at one stage or another between the 

 oosperm and the adult. Of the occurrence of both these forms of 

 variation we have direct and positive evidence. It is a familiar 

 fact that increased exercise of a part tends to an increase in the 

 bulk of its muscles. The colours and markings of certain Fishes 

 can be altered at will (of course within certain limits) by changing 

 the material on the bottom of the aquarium in which they are 

 confined ; the colours of many Caterpillars may be altered by 

 changing the colour of their surroundings. A third set of variations 

 probably also occurs, though direct evidence is wanting -namely, 

 variations which may arise within the sexual cells before the union 

 of ovum and sperm, or which may result from that union. The 

 former two sets of variations are generally spoken of as " acquired 

 characters " new characters acquired during the lifetime of the 

 individual but their nature would be more clearly indicated by 

 terming them extrinsic variations, as contrasted with the intrinsic 

 variations forming the last group. 



The extrinsic variations being brought about by the action of 

 external conditions, their causes are very various. In every such 

 case the organism responds to some persistent external influence 



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