EVOLUTION— PYCRAFT 223 



again, we have no structural evidence of burrowing. At some time 

 in the remote past, however, the ancestral mole took to a diet of 

 worms and to pursuing them, as well as grubs of many kinds, bj- 

 driving tunnels under the ground. Hence, he was committed to 

 digging for his very existence. And these intensive labors, be it re- 

 membered, fall with equal insistence on male, female, and young, 

 and unceasingly. As a consequence of the persistent stimuli to which 

 the fore limbs especially have been subjected, the characteristic short- 

 ening of the arm and forearm and the enormous hand, have come 

 into being. 



But, be it noted, there are many other animals in no way related 

 to the mole, which have to dig for a living, and in each case the re- 

 sultant modification of the fore limb is different. In Notoryctes^ 

 which is a marsupial and therefore a member of a totally different 

 group of mammals, the general form of the body is singularly like 

 that of our mole. But the fore limbs, though gi^eatly shortened, do 

 not show the great hand of the mole. The reaction to their use as 

 digging organs has brought about an enormous enlargement of the 

 claws. In no two animals, unless they be of the same species, will 

 the same organ show precisely similar responses to the same kind 

 of stimulus brought about by use, in this connection of digging. 

 The armadillos, and the anteaters, have also forefeet shaped by per- 

 sistent digging. But since they must live above ground, and not be- 

 neath it, the legs have not greatly changed from the type common 

 to creatures which must walk, and run, to get a living. 



Let me now turn to swimming. I have cited the rabbit and the 

 sand martin as burrowers, which give no evidence of this habit either 

 externally or internally. The water vole may be cited as a similar 

 example among swimming animals. We can as easily account for 

 this, as in the case of these burrowers. 



But the hippopotamus affords us a peculiarly interesting example 

 of the way "adjustments" are made where stimuli are intensive and 

 persistent. And here they are found in the head. This animal 

 passes the greater part of its life in the water, and can swim and 

 dive with great facility, and remain long submerged. Look at its 

 head, and note that the nostrils, eyes, and ears have all become 

 raised above the general level of the skull, enabling the creature to 

 breathe, see, and hear while the rest of the great body is submerged. 

 But why do the limbs show no "adjustment" to this aquatic ex- 

 istence? The explanation is simple. These great beasts have to 

 take long journeys overland, in search of fresh feeding grounds, and 

 heavy strains and stresses are placed upon the limbs when the great 

 body is no longer supported by the water. Hence, a large pro- 

 portion of their "repair tissue" is absorbed to keep up the efficiency 



