CHAP. XII The Life-History of Animals io^ 



if we ask why an animal develops a notochord only to have 

 it rapidly replaced by a backbone, part of the answer surely 

 is that the notochord which in the historical evolution supplied 

 the stimulus necessary for the development of a backbone, is 

 still necessary in the individual history for the same purpose. 



But there is no doubt that the idea of recapitulation is a 

 very helpful one, in regard to our own history as well as in 

 regard to animals, and we would do well to think of it 

 much, and to read how Herbert Spencer {Principles of 

 Biology, Lond. 1 864-66) has discussed it in harmony with his 

 general formula of evolution as a progress from the homo- 

 geneous to the heterogeneous ; how Haeckel {Generelle Mor- 

 phologie, Berlin, 1866) has illustrated it, and pithily summed 

 it up in his " fundamental law of biogenesis " {Biogenetisches 

 Grundgesetz\ saying that ontogeny (individual develop- 

 ment) recapitulates phylogeny (racial history) ; how Milnes 

 Marshall (see Nature, Sept. 1890) has recently tested and 

 criticised it, defining the limits within which the notion 

 can be regarded as true, and searching for a deeper rationale 

 of the facts than the theory supplies. 



{d) Organic Confinuity. In a subsequent chapter on 

 heredity, which simply means the relation of organic 

 continuity between successive generations, I shall explain 

 the fundamental idea that the reproductive cells owe their 

 power of developing, and of developing into organisms like 

 the parents, to the fact that they are in a sense continuous 

 with those which gave origin to the parents. A fertilised 

 egg-cell with certain qualities divides and forms a " body " 

 in which these qualities are expressed, distributed, and 

 altered in many ways by division of labour. But it also 

 forms reproductive cells, which do not share in the up- 

 building of the body, which are reproductive cells in fact 

 because they do not do so, because they retain the intrinsic 

 qualities of the original fertilised ovum, because they 

 preserve its protoplasmic tradition. If this be so, and 

 there is much reason to believe it, then it is natural and 

 necessary that these cells, liberated in due time, should 

 behave as those behaved whose qualities they retain. It is 

 necessary that like should beget like. 



