THE WONDER OF LIFE 617 



is of use in breaking through the egg-shell to Uberate the 

 young bird, which seems, in some cases at least, to be 

 very doubtful, then it is one of those structures which are 

 used only once. As every one knows, it usually falls ofi 

 soon after hatching. 



This fact suggests that it may not be a special structure 

 that has evolved on a line of its own, but the last reUc of 

 an old set of structures, retained because of its utiUty 

 while all the others have gone. Some recent observations 

 by B. Eosenstadt suggest that the egg-tooth of the upper 

 jaw and its corresponding vestige on the lower jaw, may be 

 rehcs of an ancient armature, older than the homy sheaths 

 we are famihar with on the bird's jaws. In the first place 

 they become homy in the embryo before there is any other 

 comification on the jaws. In the second place, the process 

 of horn-making in the egg tooth is difEerent from that 

 elsewhere. Each of the skin cells concerned turns wholly 

 into horn, nucleus and all, whereas in ordinary cases, as 

 in the homy sheaths that make the biU, only the mantle 

 of each cell is turned into horn. This is a technical point, 

 but it is of interest in suggesting that the egg-tooth is a 

 very ancient relic indeed. 



Whales' Hairs. — ^Let us look at the fact of whales' 

 hairs. If we could understand these, we should have a 

 master-key in our hand. The points are three. (1) The 

 ancestry of Cetaceans is unknown, but they are quadrupeds 

 and mammals none the less that the remains of the hind- 

 limbs are buried, and that their hairs are reduced to a 

 minimum. It is possible that whales were evolved from 

 scale-covered Mammals, which took to aquatic life. The 

 slight resemblance of the whale's flipper to that of the 

 extinct Ichthyosaurian reptiles cannot mean much; for 



