MODIFICATION OF SPECIES 353 



thought of as a group of individuals most of which have^most of their 

 inherited characteristics in common. Characteristics due to environ- 

 ment and differing in individuals solel}^ because of different environ- 

 mental influence are not considered. The difficult}^ in applying the 

 foregoing idea lies in the word ''most," for there is much disagreement 

 among taxonomists as to how much it should include. Probably no 

 species that can be recognized as a species anywhere in the world has 

 all of its individuals alike in all hereditary qualities. It would be possible 

 to assemble groups of individuals alike in all their genes (page 224), but 

 such assemblages would be much smaller than the ones now recognized 

 as species. To insist that species be entirely homogeneous would simply 

 multiply the number of species and would solve no problem either of 

 evolution or of classification. In practice, therefore, some heterogeneity 

 is admitted. As far as taxonomists agree on the grouping, all individuals 

 of a species have certain qualities in common; these qualities are held to 

 characterize the species. Beyond this general heritage, there are other 

 characters each of which is present in some individuals, but none in all 

 of them. A certain amount of variation thus exists among the indi- 

 viduals of every species. Some of this variation is nearly always visible 

 or otherwise capable of detection; but some of it is not seen, since it 

 consists of recessive genes scattered through the population. These 

 recessive genes, unless very numerous, are present more often in hetei'o- 

 zygotes than in homozygotes (page 227) and do not greatly affect the 

 species visibly; but they are a potential source of visible qualities in later 

 generations. 



Species do not as a rule cross with other species, though there are 

 many exceptions. Also, species tend to occupy different ai-eas from other 

 species. These are marks which help the taxonomist to recognize species 

 as distinct, and their intersterility is an important agent in making them 

 distinct. 



Origin of the Differences among Individuals. — What is*the source 

 of the minority of qualities in which the individuals of a species may 

 differ? Since a species is ordinarily descended from a single individual, 

 it would be expected, unless the ancestor had been an extremely heterozy- 

 gous organism, that its descendants would possess practically the same 

 genes throughout. The existence of a number of genes which are not 

 alike in all individuals indicates that some of the genes have changed in 

 some individuals. Such changes of genes are the mutations already 

 referred to (page 238) in the discussion of genetics. 



Mutations are not merely inventions to explain the variation within 

 species; the visible changes due to them have been witnessed again and 

 again in many animals and plants. Some of the first changes to be called 

 mutations were observed by Hugo de Vries, one of the rediscoverers of 



