EVOLUTION OF ANIMALS 549 



advance in the character of the highest forms from age to age. In the 

 oldest strata the highest types are invertebrates; from the Ordovician 

 through the Silurian, the fishes; from the Devonian to the Triassic, the 

 amphibians and reptiles ; and from the Triassic and Jurassic to the present 

 time, the mammals and birds, the more modern forms being the highest. 



Many facts of geographical distribution are explainable only on the 

 assumption of the truth of the evolutionary conception. The mar- 

 supials are evidently very primitive mammals, represented both in 

 Australia and in South America. Though fundamentally ahke, the 

 marsupials of South America and Australia are nevertheless quite dis- 

 tinct. These facts are best explained by assuming a common origin of 

 these animals and later modification after separation. 



589. Causes of Evolution. — The causes of evolution are not well- 

 understood at the present time. Neither the inheritance of acquired 

 characters nor the effect of use and disuse as direct causes of evolution 

 is accepted today. On the contrary, the causes are sought in certain 

 changes taking place in the chromosomes. Hereditary units in the 

 chromosomes, according to the present theories, must register every 

 transmissible modification. How they may be modified and thus cause 

 changes to occur is unknown; it is, however, possible that there is an 

 innate tendency for the genes to change, which results in evolution, 

 and it is also possible that hormones or other substances in the blood may 

 affect them. Recent work on the effect of X-rays upon the gametes 

 have shown that by their application structural modifications may be 

 brought about in the fruit fly, Drosophila, and that these changes are 

 heritable. Whether or not such rays are a factor in natural evolution 

 is yet to be determined. 



The appearance of a characteristic is often referred to as if it were a 

 response to a need or as if it had appeared for the purpose of adapting 

 the animal to a condition. When used by modern writers, however, such 

 expressions should be recognized as figures of speech. Of course no struc- 

 ture appears because it is needed, neither is necessity a cause of evolu- 

 tion; but if a structure does appear and is advantageous to its possessor, 

 it contributes both to its own persistence and to the perpetuation of the 

 race. 



590. Methods of Evolution. — Various theories have been put forth 

 as to the method by which evolution is brought about. The first modern 

 theory to be presented is that of natural selection, which was the one put 

 forward by Darwin and Wallace. This may be summarized as follows: 



(1) All organisms produce a greater number of young than can survive. 



(2) This results in competition for the necessities of hfe and a struggle 

 for existence. (3) All organisms tend to vary; some variations are 

 advantageous, others harmful, and still others of no moment. (4) As a 

 result of the struggle for existence favorable variations would tend to be 



