THE ORIGIN AND HISTORY OF ANIMAL LIFE 



609 



cies. Zoologists have convincing evidence for 

 the behcf that such changes are necessary 

 for the origin of new species, and in the 

 course of time, of new genera, famihcs, 

 orders, classes, and phyla. It is therefore es- 

 sential to determine the origin of inherited 

 variations and the effects of the environ- 

 ment upon them. As already noted, the 

 union of a sperm and an egg results in var- 

 ious combinations of genes, due to the 

 method of distribution of the chromosomes 

 during oogenesis and spermatogenesis, to 

 crossing over, and to the chance association 

 of ova and spermatozoa at the time of fer- 

 tilization. Inherited variations are due not 

 alone to chance combinations of genes, but 

 also to actual changes in the genes, that is, 

 to mutations. We know very little about the 

 origin of gene mutations, but environmental 

 factors such as radiation appear to be ef- 

 fective in bringing them about. 



NATURAL SELECTION 

 AND ADAPTATION 



Natural selection 



The activities of wild animals are directed 

 mainly toward maintenance of the individ- 

 ual, which is really a struggle for existence. 

 Ecologic studies are largely concerned with 

 the methods employed by different types 

 of animals in their struggles to adapt them- 

 selves to the physical and biological factors 

 in their environment. Those that can with- 

 stand the conditions in their environment 

 live, and the rest die. Darwin's theory of 

 natural selection to account for the origin 

 of species was based on this situation. He 

 pointed out that there are almost always 

 more individuals produced by members of 

 a species than the environment can support. 

 Thus any variations that better fit an in- 

 dividual for the struggle of existence will 

 lead to its survival; this concept has been 

 called the survival of the fittest. Thus na- 

 ture selected those individuals that were best 



adapted to their environment. These repro- 

 duced themselves, and their offspring pre- 

 sumably inherited the favorable variations 

 responsible for their survival. Darwin knew 

 that some variations were inherited and 

 others not, but at that time the mechanism 

 of heredity was not known. We now distin- 

 guish more clearly between heritable and 

 nonhcritable variations, and between large 

 mutations and smaller mutations. Perhaps 

 the most important point to be kept in mind 

 is the fact that new species evolve slowly, 

 and that natural selection has had hundreds 

 of millions of years in which to operate, as 

 the table on page 616 indicates. 



Darwin's theory has withstood almost a 

 centurv of criticism, and natural selection is 

 still recognized as the guiding factor in the 

 origin of species. Variations do occur; a 

 struggle for existence does take place; and 

 animals with certain types of variations 

 survive. However, we cannot emphasize too 

 strongly the fact that the heritable variations 

 on which natural selection acts are not the 

 direct result of environmental action, but 

 are due to gene mutations and chromoso- 

 mal recombinations of the individuals whose 

 ancestors were subjected to the forces of 

 natural selection. 



Adaptations 



That animals are adapted to the environ- 

 ment in which they live is obvious to any- 

 one who has taken the trouble to consider 

 their structure, physiology, and habits. In 

 general, every living thing reacts adaptively 

 to the external stimuli in the environment 

 in which the species evolved. An adaptation 

 may be structural, physiologic, or involve in- 

 herent patterns of behavior. We have noted, 

 for example, how in insects the wings are 

 adapted for flight, the legs for running, 

 swimming, or other purposes, the mouth 

 parts for biting or sucking, and the digestive 

 tract for digesting solid or liquid food; and 

 how in birds the wings and tail are adapted 

 for flight, the feet for perching, wading, or 



