ORGANISM AND ENVIRONMENT 5 



failures occur from time to time the organisms have usually means of 

 enduring them (reserve stores, resting stages, etc.). 



In proportion as the organism is unable to resist these influences 

 of the environment it is liable upon occasion to be harmed by them. 

 The process of evolution has been the development of organisms in 

 such a way as to set them free from such influences in respect of their 

 proper environments. Its results may be classed under three heads, 

 (i) Some, such as the acquirement of a cuticle or of a habit of burrow- 

 ing or of hibernation, merely fend off or avoid the action of the en- 

 vironment : these involve the least increase in the complexity of the 

 organism. (2) Others, such as the formation of organs for the ex- 

 cretion of the excess of water, provide for remedial action: in these, 

 as a rule, more complicated machinery is formed. (3) Others, such 

 as the development of a nervous system or of organs of locomotion 

 or weapons of oifence, bring it about that the action which results 

 from the receipt of stimuli is turned to the best advantage by the 

 organism: these cause a considerable, often a very great, complication 

 of the living machine. 



Thus a general outcome of evolution is the forming of more com- 

 plex, that is of " higher", organisms. But a relatively simple organism 

 may, in its proper environment, enjoy as much autonomy as in other 

 circumstances is possessed by one that is more highly organized. This 

 is notably true of many parasites. 



Some of the results of evolution, as for instance the formation of a 

 nervous system or of a cuticle, are such as to increase the independence 

 of the organism in all circumstances. Others, however, such as the 

 substitution of pulmonary for branchial respiration, or of absorption 

 for ingestion of food, are of value only in particular environments or 

 modes of life, and even unfit the organism for other ways of living. 

 Thus two distinct phenomena underlie the diversity of the Animal 

 Kingdom — an increase in the autonomy of the individual, and the 

 specialization of animals for particular modes of life. 



Every species, however good a fight it maintains, is threatened with 

 extinction owing to the continual loss of individuals, always by the 

 action of its environment and usually also by that "natural death" 

 which appears to await all organized protoplasm that is not periodic- 

 ally reorganized.^ In reproduction^ howev^er, the individual provides 

 by fission for the maintenance of its race. In the lower organisms the 

 protoplasm of the body retains ascertain plasticity, and in these there 

 is very often an asexual process of reproduction in which the new 

 construction that is necessary to organize at least one of the products 



^ See p. 27. It is possible that in some of the least highly organized 

 metazoa natural death either is no more inevitable than in protozoa or is long 

 delayed. 



