256 PRINCIPLES OF ANIMAL BIOLOGY 



Definition of Heredity. — Following tlu; account of the moclianism of 

 inheritance the term heredity may be defined with greater precision than 

 was possible at the beginning of the chapter. Heredity is the occurrence, 

 in the offspring, of the same genes that were in the parents. Definitions 

 which involve a likeness between parents and offspring have always been 

 objectionable because environmental conditions may intervene to destroy 

 such likeness. Thus, the offspring of a red-flowered Chinese primula will, 

 if reared under ordinary conditions, also produce red flowers. But if these 

 offspring are reared at a high temperature their flowers are white. He- 

 redity has not been destroyed, however, for if the plants are returned to 

 normal temperature they soon begin to produce red flowers again. A 

 definition that involved likeness of parent and offspring would imply the 

 absence of heredity in the case of the primulas. The definition requiring 

 likeness of genes in parents and offspring permits such environmental 

 effects. 



Variation. — -All the knowledge of heredity which we now possess has 

 been made possible through the fact that animals differ from one another 

 in discernible features. It is a commonplace to say that, had such 

 differences been wanting, little of the operations of heredity could have 

 been discovered. Characters might have been transmitted in precisely 

 the way they are known to be, but without individual differences the 

 mode of transmission would have remained unknown. Perhaps no curi- 

 osity concerning it would ever have existed. Variation is therefore one 

 of the cornerstones of genetics. 



Variation is due to several" causes, prominent among them hybridiza- 

 tion. It has been shown that, in an F2 generation, members of the same 

 family differ from one another merely because they contain different 

 genes. When the grandparents differ in only one character, there are only 

 two "varieties" in F2; wnen the grandparents differ in two characters, 

 the F2 generation includes four visibly different combinations, as was 

 seen in the case of rough, smooth, black, albino in guinea-pigs. When 

 many differences between the grandparents are involved, the F^ presents 

 exceedingly numerous possibilities. 



The differences involved in such cases must, however, have had an 

 origin. If it is true that animals have reached their present condition 

 by a process of evolution, a thesis that is developed in the last several 

 chapters of this book, it is inconceivable that this change has been brought 

 about merely by the recombination of genes in different ways. It is 

 inconceivable that the earliest organisms, whatever they were like, 

 contained genes for all the characters that are now possessed by their 

 descendants, only waiting to be shaken and thrown like dice to produce 

 the combinations now known. Changes must have been introduced in 

 some other way. There is, moreover, abundant evidence that changes 

 of this kind have been recognized in specific cases. A number of years 



