DARWIN'S THEORY 77 



some important part of its anatomy missing. In other cases, 

 as in some Echinoderinata, the egg can be cut into eight pieces 

 and each piece if it lives will develop into a creature just like 

 the normal form, only of proportionately reduced size. Why 

 the eggs of one form are constituted in one way, and those of 

 another in another way can only be explained on the principle 

 of heredity ; on the ground of the affinities of the parent form 

 and its position in the animal kingdom. 



Developmental Mechanics is in fact an adjunct and Dot a 

 rival to Comparative Embryology. By its means we analyse 

 an egg into its essential factors far better than we could by 

 looking at it through the microscope or even by cutting a 

 thin section of it. But no developmental mechanics will 

 enable us to explain why the feather-star loses its stalk, or 

 the young oyster its foot ; to give any rational account of 

 such occurrences we require the recapitulatory theory. If 

 then recapitulation of ancestral history should be the main 

 factor in determining development the question arises, what 

 are the other factors which modify it? The elucidation of 

 the secondary factors is an inquiry on which comparatively 

 little work has been done and on which, it is to be feared, 

 no very clear conceptions exist in the minds of many natural- 

 ists. A few of the more obvious may be briefly alluded to 

 here. The young of animals exist under two phases, termed 

 respectively the larval and the embryonic. In practically 

 all life-histories both phases occur : the life-history starts 

 by being embryonic and becomes larval before the adult stage 

 is reached. In the larval phase the young animal moves 

 actively about and seeks its own food; in the embryonic 

 phase, on the contrary, it is supplied with food either in 

 the form of yolk-grains embedded in its tissues or in the 

 form of secretions from the maternal womb; and it lives a 

 sheltered life either within an egg-shell or within the womb. 

 All the clearest and most striking evidences for recapitulation 

 occur in the larval phase : and we have every right to 

 assume that the larval type of development is primary and 

 that the embryonic phase is a secondary modification of it 

 The digestive tube of a larva must be in full functional activity, 

 ready to receive and digest food ; but that of an embryo may 

 be, and often is, a solid string or mass of cells packed with 



