742 LIFE : OUTLINES OF GENERAL BIOLOGY 



physiologically necessary result of the cleavage of an ovum when it 

 is not encumbered with too much yolk ; and it is possible to account 

 for the formation of a gastrula without dragging in the hypothetical 

 ancestral gastraea. The disc-like blastoderm familiar in birds and 

 reptiles has no probable reference to a primeval platelet or placula 

 stage in the evolution of Metazoa. 



An important criticism concerns specificity, i.e. the individuality 

 and uniqueness of every well-defined type. A fish may be identified 

 by a few scales, a bird by a few feathers. The cells lining the wind- 

 pipe of a horse are readily distinguishable from those of a dog, and 

 the palate of a land-snail from that of a periwinkle. There is pro- 

 nounced chemical individuality in species, as may be detected in 

 the milk of nearly related mammals or the juice of the grapes in 

 nearly related vines. It is most literally true that "all flesh is not the 

 same flesh". There is no doubt that increased precision of embryo- 

 logical work has disclosed the individuality or specificity of the 

 organism even in early stages of ontogeny. Thus the number of 

 chromosomes within the nucleus of a cell is, with few exceptions, 

 constant for each kind of organism, and the embryo of a mouse 

 could thus be distinguished from that of a rabbit, or that of an 

 onion from that of a lily. But a recognition of the fact that an 

 organism is from the start itself and no other is not inconsistent 

 with admitting a significant correspondence between steps in 

 individual development and steps in racial evolution. A tadpole is 

 from the first in several ways an amphibian and not a fish, and yet 

 in its two-chambered heart and branchial circulation it is for a time 

 distinctly piscine. 



One reason why the ontogenetic recapitulation of phylogeny must 

 be general, not precise, is that the successive gains made in the 

 course of racial evolution are not superimposed one upon another, 

 but are severally incorporated into the organisation and unified 

 with it. The additions from millennium to millennium are not like 

 new wings added to a house, for the tenements which we call indivi- 

 duals are continually dissolved, and there is reunification at the 

 start of each new life. Yet whatever further saving clauses may 

 have to be appended to the "recapitulation doctrine", the broad 

 fact remains that ontogeny is the making explicit of the germinal 

 organisation, which is what it is because of phylogeny. The past 

 lives on in the present in a manner peculiar to and characteristic 

 of living creatures, and it is because it is determined by the past 

 that an embryo moves towards a goal as if it had the future con- 

 sciously in view. The ages that are gone have bent the bow in the 

 plane along which the arrow of the individual flies. But ontogeny 

 must not be thought of as the uncoiling of a wound-up spring, or 

 as the unpacking of a marvellous treasure-box; it is a function of 

 the individuality which is somehow condensed within the germ-cell. 



