202 EVOLUTION IN BIOLOGY 



VI 



embryo has been observed to pass through these 

 successive evolutional stages in sundry Sponges, 

 Ccelenterates, 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 &quot; gastraea theory &quot; 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. 



&quot;Animal. . . per c.pigciusin procroatur, materiam simul attra- 

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

 . . . primum futuri corporis concrcmentum . . . prout augetur, 

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

 alias post alias natas, et online quasque suo cmergentes.&quot; l 



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, Excrcitationcs de Gcneraticnie. Ex. 45, &quot;Qusenam 

 sit pulli materia et quomodo fiat in Ovo. &quot; 



