EVOLUTION IN BIOLOGY. 295 



gastrula stage, before taking on the features distinctive 

 of the group to which it belongs. Stated in this form, 

 the u gastraea theory " of Haeckel appears to the present 

 writer to be one of the most important and best founded 

 of recent generalisations. So far as individual plants 

 and animals are concerned, therefore, evolution is not a 

 speculation but a fact ; and it takes place by epigenesis. 



" Animal . . . per epigenesin procreatur, materiam simul attra- 

 hit, parat, concoquit, et eadem utitur; formatur simul et augetur 

 . . . primurn futuri corporis concrementura . . . prout augetur, 

 dividitur sensim et distinguitur in partes, non simul omnes, sed alias 

 post alias natas, et ordine quasque suo emergentes." * 



In these words, by the divination of genius, Harvey, 

 in the seventeenth century, summed up the outcome of 

 the work of all those who, with appliances he could not 

 dream of, are continuing his labours in the nineteenth 

 century. 



Nevertheless, though the doctrine of epigenesis, as 

 understood by Harvey, has definitively triumphed over 

 the doctrine of evolution, as understood by his oppo- 

 nents of the eighteenth century, it is not impossible that, 

 when the analysis of the process of development is car- 

 ried still farther, and the origin of the molecular com- 

 ponents of the physically gross, though sensibly minute, 

 bodies which we term germs is traced, the theory of de- 

 velopment will approach more nearly to metamorphosis 

 than to epigenesis. Harvey thought that impregnation 



* Harvey, " Exercitationes de Generationc." Ex. 45, " Quaenam sit 

 pulli materia et quomodo fiat in Ovo." 



