CHARACTERISTICS: RELATION TO ENVIRONMENT 253 



mainly due to differences In the genes of which they are 

 composed. In other characteristics, differences are due 

 mainly to environmental diversities. There are all sorts of 

 Intermediate conditions between these extremes. We may 

 pass in review a number of different types of characters from 

 this point of view. 



Most of the diversities in obvious physical features In 

 most animals are the result of differences in the genes, so 

 that they are mainly matters of Inheritance rather than of 

 environment. In Drosophila the forms of the parts of the 

 body, of the legs and wings, the distribution of bristles, the 

 venation of the wings, the colors of body and eyes, are all 

 changeable through the action of diverse genes, but di- 

 versities In these features due to environmental action are 

 relatively rare. This is the case also for such characters In 

 the vertebrates, including man. Most of the physical char- 

 acters of human beings — form of their features, complex- 

 ion, color of hair, form of limbs, and the like, as well as 

 sex — are settled mainly while the child is carried In the moth- 

 er's body, when the differences in environment are very 

 slight. The diversities between individuals in these respects 

 are almost entirely — though not quite entirely — the result 

 of differences in the genes; they are matters mainly of in- 

 heritance. This is demonstrated by what we find In twins. 

 The one-egg twins, having the same genes, are usually 

 closely alike in their physical peculiarities, while the twins 

 derived from different eggs, and so having different genes, 

 are much more diverse with respect to such physical char- 

 acteristics. At times of course one-egg twins become diverse 

 through accidents or diseases which affect one and not the 

 other. But physical diversities in twins due to genetic dif- 

 ferences are much more frequent than those due to environ- 

 mental differences. 



In some organisms the gross physical features are read- 

 ily altered by environmental diversities, so that differences 



