xii EDITOR'S INTRODUCTION 



once she has reached out her hand to sister branches of 

 biology. 



Morphology's central conception, Homology, is being 

 modified by Genetics. Identical but independent mutation 

 of genes, as is now recorded from several different species 

 of Drosophila, shows that the conception of a common 

 ancestor is no longer fundamental to the idea of homology. 



Here we obviously approach orthogenesis. On the other 

 hand, other orthogenetic ideas derived directly from mor- 

 phological study melt away in the light of developmental 

 physiology. Such phenomena as the progressive phylo- 

 genetic horn-development of various mammals, sometimes 

 occurring independently in parallel stocks, need not after all 

 imply orthogenesis in its strict sense of steady, determinate 

 change of the germ-plasm. A study of the mechanism 

 of the relative growth of parts shows, as Mr. de Beer 

 points out (Chap. XLIII), that natural selection for increased 

 size will automatically bring out the horn-growth, as what 

 Darwin called a correlated variation. 



Recapitulation too must be viewed differently as the result 

 of studies on growth and on genetics. As D'Arcy Thompson 

 pointed out in his Growth and Form, differences in propor- 

 tion between related animals must be due primarily to differ- 

 ences in the growth-rates of the parts concerned. Later work 

 has shown that the characteristic proportion of a part gene- 

 rally depends on the part continuing to grow at a different 

 rate from the rest of the body for a long period. This being 

 so, many cases of recapitulation are due solely to this differen- 

 tial growth. Schultz has shown that the foetuses of primates 

 still show the limb-proportions characteristic of their adults, 

 but less strongly marked. This is recapitulation : but it 

 is also a direct consequence of long-continued differential 

 growth. The same principles can be applied to the recapitu- 

 lation of shell-form shown in the ontogeny of many Nauti- 

 loids, Ammonites, etc., and to the fact that vestigial organs 

 are often of greater relative size in young stages. 



Such studies also bear on the systematic side of morph- 

 ology ; for where the growth-rates of two parts are markedly 



