xv THE PHILOSOPHY OF ZOOLOGY 621 



substance can be shown to be widely diffused throughout the 

 plant. Thus in many flowering plants, if we cut a shoot into 

 lengths the pieces are all capable of giving rise under suitable 

 treatment to complete plants with flowers containing reproductive 

 cells, and in many cases a leaf, or a portion of one, is capable of a 

 similar development. In many animals a similar wide distribution 

 may be shown to prevail. This appears most strikingly in forms 

 that multiply by budding. In Hydra, for example, any part of the 

 body seems capable of giving off buds, and in the buds, after they 

 have become separate, ova and sperms are developed from the cells 

 of the ectoderm. A similar phenomenon is to be observed in other 

 Coelenterates and in the Polyzoa and the Composite Ascidians, 

 and also in certain cases among the Platyhelminthes and Annulata. 

 In all these, and other cases that might be mentioned, the germinal 

 substance is not confined to the reproductive cells, new repro- 

 ductive 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 substances. Many 

 members, not only of the lowest pyla, 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 

 or the tail. In the highest Vertebrates there is no power of 

 regenerating a lost limb or tail, 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 



