GENERAL ZOOLOGY 



organized and delicately balanced functional mechanisms, about which the 

 structural features are organized. Such systems obviously cannot be changed 

 in completely random ways, or with any large deviations from the normal, 

 without disrupting their fundamental conditions of equilibrium. The char- 

 acteristics of organisms are thus bounded at all stages by rather finite limits, 

 and any changes beyond these limits are detrimental to the organism. To 

 this extent, then, heritable variations or mutations cannot occur at random. 

 For any particular kind of organism, the number of directions in which 

 change can proceed, consonant with continued life, is limited; and only such 

 changes as may proceed in these directions can persist and give rise to new 

 evolutionary stocks. Changes in some directions may, under certain environ- 

 mental conditions, be of survival value to the organism and thus may be 

 selected for preservation. Changes in other directions may appear to be non- 

 adaptive, or to have no demonstrable survival value; these may persist 

 because they are "neutral," because selective processes do not operate 

 to eliminate them, or because they have become genetically linked with 

 selected characteristics. This may account for the persistence, through long 

 evolutionary sequences, of such apparently non-adaptive characteristics as 

 overgrown spines and large, complicated shells among some of the inverte- 

 brate groups. Changes which are adaptive, and thus selected, in some stages 

 of the history of a group may, under changed conditions, become neutral; or 

 they may actually become detrimental and so contribute to the eventual ex- 

 tinction of the race. The evolution of non-adaptive characteristics, and 

 the evidence that evolution is not a completely random process, may be ex- 

 plained by some such reasonable considerations. 



Summary 



The history of organisms, as indicated by the data of biology and other 

 sciences, has involved gradual processes of change from some primitive 

 form or forms of life. How and when these primitive forms originated on our 

 planet are matters of speculation. The place of origin was probably in the 

 primordial seas, and the time must have been some period after these waters 

 had cooled sufficiently to permit living systems to exist. Evidence drawn from 

 a wide range of observations and experiments demonstrates the reality of the 

 evolutionary changes that have produced existing forms from this primitive 

 ancestry. The facts of distribution, both geologic and geographic, and the 

 facts of anatomy, embryology, and physiology can be most reasonably ex- 

 plained in accordance with the theory of organic evolution. The strength of 

 this evidence is in its extent and diversity. Any other explanation of the 

 data is a violation of common sense as well as of scientific reasoning. Much 

 of the evidence is indirect and circumstantial, but more direct and experi- 

 mental evidence, based on studies in plant and animal breeding, genetics, and 

 ecology, confirms the conclusion that the only reasonable explanation for a 



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