202 EVOLUTION IN BIOLOGY vi 



embryo has been observed to pass through these 

 successive evolutional stages in sundry Sponges, 

 Coelenterates, Worms, Echinoderms, Tunicates, 

 Arthropods, Mollusks, and Vertebrates ; and there 

 are valid reasons for the belief that all animals of 

 higher organisation than the Protozoa agree in the 

 general character of the early stages of their indi- 

 vidual evolution. Each, starting from the condition 

 of a simple nucleated cell, becomes a cell-aggregate ; 

 and this passes through a condition which re- 

 presents the gastrula stage, before taking on the 

 features distinctive of the group to which it belongs. 

 Stated in this form, the " gastraea theory " of 

 Haeckel appears to the present writer to be one of 

 most important and best founded of recent general- 

 isations. So far as individual plants and animals 

 are concerned, therefore, evolution is not a specu- 

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



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

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

 . . . primum futuri corporis concrementum . . . prout augetur, 

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

 alias post alias natas, et ordine quasque suo emergentes." 1 



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. 



1 Harvey, Exercitationes de Generatione. Ex. 45, "Quaenam 

 sit pulli materia et quomodo fiat in Ovo. " 



