268 EVOLUTION AND GENETICS 



Evolutionary Tendency. The effect of variations in evolution 

 becomes evident only through the study of other related phe- 

 nomena. It is impossible to determine this quality of variations 

 by the examination of an existing species alone, but complete 

 information about a phylogenetic series often discloses that varia- 

 tions in the past have followed either of two tendencies through 

 successive generations. They have been either fortuitous or 

 orthogenetic. 



Fortuitous Variations. These are the ordinary fluctuating 

 variations which appear generation after generation apparently 

 always within the same limits and about the same mean. There 

 is no available evidence to prove that they contribute to the 

 evolution of species, although positive proof that they take no 

 part in evolution is equally lacking. 



Orthogenetic Variations. Such variations are evident in the 

 field of paleontology. Their trend is in a definite direction through 

 a phylogenetic series, toward the ultimate modification of the 

 species in which they occur. A striking case of orthogenetic 

 variation is the gradual succession of changes in the foot of the 

 horse from Eohippus to the modern genus Equus as described in 

 chapter X. 



Heritability. A more fundamental classification of variations 

 from the point of view of genetics is based on their heritability. 

 They are divided into three groups. 



Modifications. Modifications are changes which appear in the 

 individual during the ordinary course of its life. They are usu- 

 ally looked upon as a product of environment, but should be 

 regarded, with very few exceptions, as the product of inherited 

 powers responding to environmental conditions. Such are tanning 

 of the skin and muscular development. It is evident that these 

 characters are not inherited as they develop in the individual, 

 but must develop anew with each generation if the proper con- 

 ditions are encountered. For the purposes of genetics they may 

 therefore be looked upon as not heritable.^ 



Combinations. Since no two individuals are exactly alike, it 

 follows that biparental reproduction will mix in the second genera- 



1 Modifications are usually regarded as the effects of environment on organisms. 

 Few characters can be caused wholly by the environment, however, and most of the 

 so-called modifications are as described above. For this reason it seems desirable 

 to retain the term as it is used here. The question is considered further in chap- 

 ter XXIV. 



