THE EVOLUTION OF ANIMAL LIFE 



tions made up orie;inallv of gencticallv homoe^cncous and Irccly intcrhrccdini^ 

 individuals, separate small groups maintained in isolation for long periods 

 of time would undoubtedly in the long run develop into distinct species, just 

 as these snails of Oahu have done. 



There are many other examples of the effectiveness of such geographical or 

 ecological isolation. Sometimes, isolating mechanisms operate even when 

 adult forms occur together. These mechanisms may be psychological, as 

 when for anv reason males and females of the two forms fail lo show se.xual 

 interest in each other; they mav be mechanical, involving a physical incom- 

 patibilitv which prevents copulation; or they may be physiological, involving 

 failure of spermatozoa to reach the ova to effect fertilization. In still other 

 forms, hybrid offspring are inviable, dying at some stage of development be- 

 fore they reach sexual maturity. Hybrids that survive to the stage at which 

 reproduction occurs in the parent species are commonly incapable of pro- 

 ducing viable gametes. Mechanisms of isolation, particularly those involving 

 reproductive functions, are undoubtedly related to the establishment of 

 incipient species by the formation of subspecies or lesser groups that breed 

 successfully only among themselves; at later stages they are important in 

 maintaining the boundary lines between established species. 



This origin of incipient species must be recognized as the next stage in 

 evolution after the appearance of heritable variations. It is possible to 

 understand, even without an extended explanation of the genetic principles 

 involved, that the appearance of heritable variations (changed gene com- 

 plexes) and their perpetuation, first in small groups of individuals and then 

 in larger groups which eventually become species, are the first steps in 

 evolution. Thus, one species may arise from a pre-existing species, and a 

 third from the second, and so on; and great evolutionary changes may occur 

 in the course of time. Since the origin of life on earth, there has certainly 

 been enough time to permit the operation of such mechanisms of evolution. 

 In the last analysis, as Darwin clearly recognized, accounting for the 

 origin of species is the key to an understanding of the evolutionary process. 



It has been argued that this view of evolutionary mechanisms and of the 

 importance of natural selection is inadequate, because it fails to explain 

 many lines of evidence that seem to indicate that mutations and subsequent 

 evolutionary changes do not occur at random and in all directions. The 

 evidence often cited involves evolutionary sequences in which changes all 

 seem directed toward some pre-determined end form, and in which it is pos- 

 sible to detect steady trends, such as increase in size and the consistent 

 development of special characteristics which appear to be non-adaptive or 

 without obvious selective value to the organism. From the standpoint of 

 genetics, it is argued that the occurrence of mutations is not a chance 

 phenomenon, because, for example, the same mutations occur repeatedly in 

 populations of organisms. All this evidence has been taken to indicate the 

 existence of what has been termed "plan and purpose" in evolution. 



It must be borne in mind, however, that living systems involve specifically 



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