ANIMAL PEDIGREES 203 



elaborated by many, notably by Balfour and Ernst 

 Haeckel. 



A few illustrations from different groups of 

 animals will best explain the practical bearings of 

 the theory, and the aid which it affords to the 

 zoologist in his attempts to reconstruct the pedigrees 

 of animals ; while these will also serve to illustrate 

 certain of the difficulties which have arisen in the 

 attempt to interpret individual development by the 

 light of past history ; difficulties which I propose to 

 consider at greater length. 



A very simple example of recapitulation is 

 afforded by the eyes of the sole, flounder, plaice, 

 turbot, and their allies. These " flat fish " have 

 their bodies greatly compressed laterally, and the 

 two surfaces, really the right and left sides of the 

 animal, unlike, one being white or nearly so, and 

 the other coloured. The flat fish has two eyes, 

 but these, in place of being situated as in other 

 fish one on each side of the head, are both on the 

 coloured side. The advantage to the fish is clear, 

 for a flat fish when at rest lies on the sea bottom, 

 with its white surface downwards and the coloured 

 one upwards. In such a position an eye situated 

 on the white surface could be of no use to the fish, 

 and might even become a source of danger, owing 

 to its liability to injury from stones or other hard 

 bodies on the sea bottom. 



No one would maintain that flat fish were 

 specially created as such. The totality of their 

 organisation shows clearly enough that they are 

 true fish, akin to others in which the eyes are 



