THE PRESENT POSITION OF THE THEORY OF ORGANIC EVOLUTION. 107 



the evolution of the horse out of a mammal having four toes on 

 the fore limb and three on the hinder limb, like the modern tapir. 

 Now in this series we see no evidence of the sudden acquisition or 

 the sudden dropping of a " factor " ; rather the change seems to 

 have been due to the increase of certain parts by use and the 

 diminution of other parts by disuse. Again, in the Karoo 

 desert of South Africa there is a series of beds representing an 

 even greater lapse of time, and in these we have the record of the 

 evolution of a mammal out of a reptile. Here again no evidence 

 of mutations in the original sense is seen, but in such important 

 matters as the arrangement of the jaw-bones and ear-bones, 

 wherein the difference between a mammal and a reptile is most 

 marked, evidence of gradual change in size coincident with 

 change in function is seen. 



If Mendelism fails to suit the facts of palaeontology, still 

 more is it in disaccord with the facts of embryology. It is often 

 tacitly assumed by Mendelians, who work chiefly with mam- 

 mals and with the higher plants, that the young form is pro- 

 duced with all the characters of the adult. But, of course, in a 

 large number of animals this is not so : the young one begins life 

 as a larva which, in form and habits, is unlike the adult, and 

 which only gradually acquires the form of the adult as it 

 assumes the habits of the adult. Now, it has been found, if we 

 take the case of an aberrant member of a group in which the 

 normal type of adult structure is fairly constant, that the 

 aberrant member when young exhibits a type of structure much 

 more like the normal type than it does when it is adult. This 

 phenomenon is interpreted in this way : the aberrant member 

 of the group is supposed to have taken up a new mode of life 

 and to have had its structure changed in consequence as a reaction 

 to the new mode of life — just as continued exercise makes the 

 leg muscles of the athlete increase in size. This reaction, in 

 course of long generations, is believed to have been fixed in the 

 constitution of the germ, so that eventually it comes about 

 before the new environment has had time to act. A beautiful 

 example of this is given by the life-history of the hermit crab. 

 This crustacean, when adult, protects its abdomen by thrusting 

 it into the empty shell of a whelk or sea-snail, and the abdomen 

 becomes curved in conformity with the curvature of the shell. 

 But when the hermit crab is young, its abdomen is quite 

 straight, like that of other crabs and lobsters. If, now, a young 

 hermit crab be reared to maturity, but be prevented from 

 finding a shell, its abdomen will become curved, although not 

 so much curved as if it had found a shell. We mi^ht well ask 



