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 “ gastrea 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 epigenesin procreatur, materiam simul attra- 
hit, parat, concoquit, et eddem 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.” } ; 
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, Hxercitationes de Generatione. Ex. 45, ‘ Quenam 
sit pulli materia et quomodo fiat in Ovo.” 
