FISHES 163 



number. Hence if selection ceases to weed out the 

 one group, all the animals will come to reproduce and 

 cross with each other ; in the general mixture the plus 

 and minus will neutralise each other. The larger 

 variations cannot prevail in the succeeding generations, 

 because they are affected by just the same number of 

 smaller variations. An organ that is subjected to 

 panmixis will thus become neither smaller nor larger, 

 but remain of the same size. It will become one of 

 what are called the indifferent marks of the particular 

 species a mark that has no connection with the 

 essential life of the animal, yet is tenaciously retained in 

 the structure. Are there any of these indifferent marks 

 in the organic world ? 



In the first place, it is always precarious to describe 

 certain characters of animals as indifferent. In earlier 

 years all that we have recognised to-day as adaptation 

 was regarded as a mere mark of the species. Every 

 year there are fresh discoveries of adaptations ; we are 

 constantly finding an important vital significance in 

 parts of animals that had been regarded as of no 

 consequence whatever, 



Nevertheless, they may be indifferent organs, and 

 we can easily imagine how they may arise. When one 

 species is formed from another, a whole series of organs 

 are modified to meet the new conditions. But the new 

 species will also inherit from the parent species features 

 that were necessary to the ancestors in their particular 

 conditions, but have no use in the new environment. 



If these organs were in the way of the new life, they 

 would, like the gills of the fish and the fingers on the 



