468 the president's address. 



cells all remain together instead of separating. Somehow or other 

 they have learnt the value of co-operation. At the close of 

 segmentation, in a typical animal development such as that of a 

 sea urchin, or even of so highly organised a form as Amphioxus, 

 the embryonic cells arrange themselves in a form which exactly 

 reproduces the arrangement seen in a colony of Volvox, giving rise 

 to the blastula, a hollow spherical embryo with a wall composed 

 of a single layer of cells. 



It seems fairly certain, from such considerations, that the origin 

 of all the higher animals is to be found in the habit, in which 

 so many Protozoa indulge, of forming colonies, and the particular 

 type of colony which has led to the best results appears to have 

 been that adopted by Volvox and by the radiolarian Sphaerozoum. 

 The branching type of colony, met with in the Vorticellidae and 

 many other groups, appears to have led to no important advance 

 in organisation, and we shall see presently that this holds true 

 also in the case of higher divisions of the animal kingdom. 



The mere habit of colony formation is not, however, sufficient 

 to secure progress : there must also be differentiation and division 

 of labour amongst the constituent cells, so that the entire 

 organism may form a machine of greater efficiency in the 

 struggle for existence. In this process the individual cells 

 become mutually dependent upon one another the whole 

 colony undergoes what is termed integration, and comes to form 

 a single individual of a higher order, an individual which cannot 

 be separated into its constituent parts without perishing. 



In such forms as Sphaerozoum, Volvox, and the blastula stage 

 in the development of higher animals, the processes of differentia- 

 tion, division of labour and integration have not gone very far, 

 and such forms may still be regarded as mere colonies of single 

 cells. In the main line of evolution of the animal kingdom the 

 next step appears to have been the conversion of the hollow 

 spherical colony of Protozoa into the coelenterate type, a process 

 which is represented in every typical development by the conver- 

 sion of the blastula into the gastrula. 



If we accept the familiar principle of the recapitulation of 

 ancestral history in individual development, we gain a very clear 

 idea as to how the coelenterate type probably arose. The hollow 

 sphere of one layer of cells became converted into a sac formed of 

 two layers, with a mouth at one end leading into a primitive 



