112 GENERAL ZOOLOGY 



organs, but this is the condition we should expect, according 

 to the mutation theory. 



Professor Morgan brings his discussion to a close by a 

 statement contrasting the mutation theory with the selec- 

 tion theory, as follows : "Animals and plants are not changed 

 in this or in that part in order to become better adjusted 

 to a given environment, as the Darwinian theory postulates. 

 Species exist that are in some ways very poorly adapted to 

 the environment in which they must live. If competition 

 were as severe as the selection theory assumes, this imper- 

 fection would not exist. 



" In other cases a structure may be more perfect than the 

 requirements of selection demand. We must admit, therefore, 

 that we cannot measure the organic world by the measure of 

 utility alone. If it be granted that selection is not a mold- 

 ing force in the organic world, we can more easily understand 

 how both less perfection and greater perfection may be present 

 than the demands of survival require. 



" If we suppose that new mutations and 4 definitely ' inher- 

 ited variations suddenly appear, some of which will find an 

 environment to which they are more or less well fitted, we 

 can see ho^w evolution may have gone on without assuming 

 new species have been formed through a process of competi- 

 tion. Nature's supreme test is survival. She makes new 

 forms to bring them to this test through mutation, and does 

 not remodel old forms through a process of individual 

 selection." 



MendePs Law of Heredity. Considerable attention has 

 lately been given to the experimental study of heredity and 

 variation by breeding animals and plants under close observa- 

 tion. Among the animals thus observed have been mice, 

 guinea-pigs, and rabbits. Little has been done along this 

 line with the insects. These experiments have been of great 

 interest in connection with what is known as Mendel's law. 



