362 THE VARIATION OF ANIMALS IN NATURE 



presented by imperfect co-ordination of the individuals within 

 the species. 



Nowhere do living animals show their characteristic 

 organisation more conspicuously than in the course of their 

 early development. In many species the early developmental 

 phenomena (e.g. types of cell cleavage) would seem to be 

 needlessly elaborate, but if the processes are followed through 

 to their end, each step can be seen to lead logically to the final 

 organisation. Experimental studies have shown, also, that in 

 the early stages there is a considerable power of forming 

 a perfect organism in spite of interference with the normal 

 course of events. These facts have been so much discussed 

 recently that we need not enlarge on them. We may, 

 however, refer briefly to the controversy as to how far develop- 

 ment is a purely ' physico-chemical ' process. From one point 

 of view it is obvious that development is not merely a series 

 of physico-chemical reactions : chemical reactions, however 

 complicated, are not known to produce such organised systems 

 as living animals. It is probable that each stage in development 

 obeys a system of physico-chemical laws, but this does not 

 imply that development is merely a chain of reactions which 

 follow one another automatically. The regulation of the 

 reactions so that each produces a desired result, no more and 

 no less, is characteristic of organisms but not of unorganised 

 chemical processes. Further, each organism forms part of a 

 continuous series, and it is logically unsound to single out part 

 of the series and regard it as a whole. Thus, even if it were 

 maintained that the development from egg to adult is merely 

 a chain of chemical reactions, it is still necessary to explain 

 how the egg came to be in a situation where develop- 

 ment was possible. We find, then, that at the start a 

 system of organised internal relations is the fundamental, 

 almost axiomatic, assumption in any definition of a living 

 organism. 



The automatic and self-regulating quality of animals is no 

 less conspicuous in the life of the adult, especially in the more 

 highly evolved forms. Thus Haldane (1929), dealing with 

 the failure of purely mechanistic explanations in physiology, 

 instances the phenomena of heredity and of regeneration as 

 showing the tendency of living organisms to reach and main- 

 tain a stable form. 



