58 I The Process of Evolution 



its properties through time and interacting with the nuclear mate- 

 rial and the environment (including other cells). A unicellular or- 

 ganism is, in a way, immortal. The "end" of a cell usually is the 

 result of accidental destruction (including the result of predation) 

 or the division of one cell into two. Death comes eventually to the 

 somatic cells of multicellular creatures and may be considered a part 

 of the genetic system. 



Increase of size, or growth, is inherent in the idea of continuing 

 reproduction. The term development refers to the changes that take 

 place during the life of an organism. Simple changes in surface- 

 volume relationships, which may have constituted development in a 

 protoorganism, seem a far cry from the life cycle of a monarch 

 butterfly (egg-larva-pupa-adult), but the difference is one of 

 degree, not of kind. Organisms change size in growth, and what is 

 a working design at one size may be completely nonfunctional at 

 another. Given the physical limitations of the size of mammalian cells 

 (imposed by such factors, among many others, as the size of protein 

 molecules and rates of diffusion), it is easy to see that a perfect 

 miniature human the size of an ovum, or a sperm cell 6 feet long, 

 would be impossible. A genetic mechanism thus does not ensure the 

 production of duplicates of the parental multicellular organism but 

 rather the production of entities that, within certain limits of varia- 

 tion, will develop into replicates of the parental type. 



This regulation may be termed developmental homeostasis and is 

 closely tied to the concept of "wild type." Most kinds of organisms 

 seem to vary greatly only in rather superficial characteristics. The 

 Drosophila wild type has been extensively described genetically. 

 The human wild type does not have each eye of a different color or 

 six digits on each hand. Redwood trees may vary in height and 

 branch number, but they have characteristic green leaves of distinc- 

 tive arrangement and rough red bark. One is more readily struck 

 with variation in large organisms than in small organisms, but this 

 does not necessarily mean that small organisms are less variable. In 

 all organisms, it seems to be true that critical developmental systems 

 are relatively immune to genetic alteration. It is advantageous for 

 an organism to avoid reproductive waste by producing optimum 

 phenotypes from a number of minor variant genotypes. In many 

 organisms the processes of development of a specific form have be- 

 come canalized, leading to a uniform phenotypic expression of in- 

 dividuals in a given population in spite of the genetic variability 

 among them. With this mechanism, genetic variability (of long- 

 range importance) can be present with a minimum of reduction of 

 fitness. 



