384 V. THE NATURE OF EVOLUTION 



the individual thus selected must have the property to develop still 

 iurther the character in question, the character will be advanced un- 

 limitedly in so far as the environmental effect causing the change 

 continues to exist, thus resulting as a natural result in overspeciali- 

 zation or overgrow^th, in which once useful parts are carried entirely 

 beyond the point of utility until they become iujurious and possibly 

 ■even instrumental in the extinction of the species. The tusks of the 

 saber-toothed tiger, the antlers of the extinct Irish elk, the huge size 

 of certain dinosaurs, and the tusks of the Columbian elephant are 

 often-cited examples of such disadvantageous development. 



As already discussed, although the genes seem to direct each 

 particular restricted structure of protoplasm protein, the change at a 

 certain structural portion of a protein molecule by a gene tends to 

 ^xert influences upon other portions, and accordingly a development 

 of a useful, selected character is inclined commonly to be associated 

 with a development of certain other characters which may not be so 

 useful or even utterly useless. In addition, as will be mentioned 

 later in detail, hormones play a predominant part in the evolution in 

 developing many characters by their peculiar effect upon genes. These 

 may be the reasons why organs appearing to be of no use are also 

 often overspecialized. 



In fact, useless characters, along with those that are useful, seem 

 to be common in organisms. This is particularly true for the struc- 

 tural features of animals. In recent years a number of mass-selection 

 experiments directed at increasing, as well as decreasing, the magni- 

 tude of a given quantitative character have been reported. A constant 

 feature of such experiments has been the occurrence of correlated 

 changes in characters other than the one being selected (67). The 

 difficulty of the independent occurrence of a change restricted to a 

 portion of the protein molecule is shown also in the fact that the 

 influence of genes is believed to be usually dependent upon their 

 association with certain other genes. Evidences for this interaction 

 of genes is clear-cut. At least twenty five pairs of allelomorphic 

 genes are believed to be concerned with eye-colour. Conversely, a 

 single pair of genes may influence more than one character. In many 

 instances it is known that the three or more allelic states of the 

 genes for a particular character exist. A single pair of genes may 

 .also influence the appearance of more than one character. It has 

 also been known that genes have somewhat different actions accor- 

 ding to what neighbours they possess. This is the so-called position 

 •effect. 



Anyhow, the individual having an extreme character shares the 

 ^ene which will more and more extend the extreme character ; conse 



