xv THE PHILOSOPHY OF ZOOLOGY 059 



among the Platyhelrninthes and Annulata. In all these, and in 

 other cases that might be mentioned, the germinal substance is not 

 confined to the reproductive cells new reproductive cells being 

 capable of being formed from the substance of the cells of various 

 tissue-layers. 



The phenomena of regeneration are important in connection 

 with this question of the site of the germinal substance. Many 

 members, not only of the lowest phyla, but of the Echinodermata, 

 the Annulata, the Arthropoda, the Mollusca and the Chordata, 

 are able, as has been repeatedly mentioned, to replace by a 

 process resembling budding, parts that have been broken off: some 

 of 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 regene- 

 ration, 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 present- 

 ing 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 



