308 THE PHILOSOPHY OF BIOLOGY 



The states of this system are (i) the gaseous state in 

 which the molecules of the water are moving at 

 a high velocity and are a relatively considerable dis- 

 tance apart, and in which they are incessantly colliding 

 with each other and with the walls of the vessel ; 

 (2) the state of the system consisting of the separate 

 phases, liquid water and gaseous steam in contact 

 with it ; and (3) the solid phase, in which the 

 molecular motions almost, or quite, cease. Here the 

 progress of the system through its phases leads to 

 physical diversity and then again to physical homo- 

 geneity. But the diversity of the different phases is 

 in a sense an apparent one only : any single phase, or 

 at least those which involve the passage of the system 

 from the gaseous to the liquid phases, and vice versa, 

 can be represented by van der Waal's general equation, 



RT = (p + -3 ) (v-b). Does anything in modern biological 



investigation, except, of course, the speculations of 

 non-physical physiologists, suggest that an ontogenetic 

 process can be represented in such a manner ? 



Are the arbitrary " stages " of the embryologists 

 the ovum, blastula, gastrula, etc., phases in a system in 

 the above sense, the only sense in which the process can 

 be regarded as capable of physico-chemical analysis ? 

 What precisely is the embryo at the close of the process 

 of segmentation ? It is an harmonious equipotential 

 system, that is to say, an assemblage of discrete organic 

 parts or cells, each of which has all the potentialities 

 that every one of the others has. Any cell in the 

 blastula may become a cell, or a series of such, in any 

 part of the gastrula or pluteus larva. This is what the 

 parts are in potentiality, but actually their individual 

 fates are different. The system is an harmonious one, 

 and each of its parts, although able to do whatever 



