MERISTIC PHENOMENA 41 



cells were always dissimilar — that is to say, if differentiation 

 always occurred — we could conceive some rough comparison 

 with such dissociations. But we know the dissimilarity between 

 daughter-cells is not essential. In the reproduction of unicel- 

 lular organisms and many other cases, the products formed at the 

 two poles are, so far as we can tell, identical. Any assumption 

 to the contrary, if we were disposed to make it, would involve 

 us in difficulties still more serious. At any rate, therefore, if 

 differentiation be really the central difficulty in development, 

 it is division which is the essential problem of heredity. 



Sir George Darwin and Professor Jeans tell us that " gravita- 

 tional instability" consequent on the condensation of gases is 

 "the primary agent at work in the actual evolution of the uni- 

 verse," which has led to the division of the heavenly bodies. The 

 greatest advance I can conceive in biology would be the dis- 

 covery of the nature of the instability which leads to the c ntinual 

 division of the cell. When I look at a dividing cell I feel as an 

 astronomer might do if he beheld the formation of a double 

 star: that an original act of creation is taking place before me. 

 Enigmatical as the phenomenon seems, I am not without hope 

 that, if it were studied for its own sake, dissociated from the 

 complications which obscure it when regarded as a mere incident 

 in development, some hint as to the nature of division could be 

 found. It is I fear a problem rather for the physicist than for 

 the biologist. The sentiment may not be a popular one to utter 

 before an assembly of biologists, but looking at the truth imper- 

 sonally I suspect that when at length minds of first rate analytical 

 power are attracted to biological problems, some advance will 

 be made of the kind which we are awaiting. 



The study of the phenomena of bodily symmetry offers 

 perhaps the most hopeful point of attack. The essential fact 

 in reproduction is cell-division, and the essential basis of heredi- 

 tary resemblance is the symmetry of cell-division. The phenom- 

 ena of twinning provide a convincing demonstration that this 

 is so. By twinning we mean the production of equivalent struc- 

 tures by division. The process is one which may affect the whole 

 body of an animal or plant, or certain of its parts. The term 



