328 The Study of Animal Life part iv 



logically the difficulty is not greater. A fertilised egg-cell 

 with qualities abcxyz divides into many cells, which, becom- 

 ing diverse, express the original qualities in various kinds 

 of tissue within the forming body. But if at an early stage 

 certain cells are set apart, retaining the qualities or charac- 

 ters abcxyz in all their entirety, then these, when liberated 

 after months or years as egg-cells, will resemble the original 

 ovum, and are able like it to give rise to an organism, 

 which is necessarily a similar organism. 



To call heredity " the relation of organic continuity 

 between successive generations," as I define it, seems a 

 truism to some, but it is in the realisation of this truistic 

 fact that the modem progress in regard to heredity consists. 



To ask how the inherent qualities of the ovum become 

 divergent in the different cells of the body, or how some 

 units remain embryonic, or how the egg-cell divides at 

 all, is to raise the deepest problems of biology, not of 

 heredity. To answer such questions is the more or less 

 hopeless task of physiological embryology, not that of the 

 student of heredity. Recognising the fact of organic con- 

 tinuity, various writers such as Samuel Butler, Hering, 

 Haeckel, Geddes, Gautier, and Berthold, have sought in 

 various ways to make it clearer, e.g. by regarding the re- 

 production of like by like as an instance of organic memory. 

 As these suggestions are unessential to our argument, I 

 shall merely notice that there are plenty of them. 



How far has this early separation of the future repro- 

 ductive cells from the developing body been observed ? It 

 has been observed in several worm-types — leeches, Sagitta, 

 thread-worms, Polyzoa, — in some Arthropods {e.g. Moina 

 among crustaceans, Chironomus among Insects, Phalangidas 

 among spiders), and with less distinctness in a number of 

 other organisms, both animal and vegetable. In most, of 

 the higher animals, however, the future reproductive cells 

 are not observable till development has proceeded for some 

 days or weeks. To explain this difficulty, Weismann has 

 elaborated a theory which he calls " the continuity of the 

 germ-plasma!' The general idea of this theory is that of 

 organic continuity between generations, and this Weismann 



