88 THE WHENCE AND THE WHITHER OF MAN 



assigned to the formerly popular argument of the 

 "missing links." The wonder is not that so many 

 links are missing, but that any of these primitive forms 

 have come down to us. For we see here another proof 

 of the fearful extermination of lower forms during the 

 progress of life on the globe. It seems as if the inter- 

 mediate forms were less common among these most 

 recent animals than among the older types. This 

 may not be true, for it is not easy to compare the gap 

 between two mammals with that between two worms 

 or insects, and mistakes are very easily made. But it 

 seems as if extermination had done its work more ruth- 

 lessly among these highest forms than among their 

 humbler and lower ancestors. I would not lay much 

 weight on such an opinion ; but, if true, it has a mean- 

 ing and is worthy of study. 



In higher, true, placental mammals the period of 

 pregnancy is much longer, and the young are born in 

 a far higher stage of development, or rather, growth. 

 The stage of growth at which the young are born dif- 

 fers markedly in different groups. A new-born kitten 

 is a much feebler, less developed being than a new- 

 born calf. An embryonic appendage, the allantois, 

 used in reptiles and birds for respiration, has here 

 been turned to another purpose. It lays itself against 

 the walls of the uterus, uterine projections interlock 

 with those which it puts forth, and the blood of the 

 mother circulates through a host of capillaries separ- 

 ated from those of the blood system of the embryo 

 only by the thinnest membrane. This is the placenta, 

 developed, in part from the allantois of the embryo, 

 in part from the uterus of the mother. It is not a new 

 organ, but an old one turned to better and fuller use. 



