THE FINAL ADJUSTMENTS IO9 



young embryos are very difficult to find, and as yet 

 no stage is known which has not burrowed into the 

 side of the uterus, digesting the maternal tissue so 

 as to prepare a space in which to develop embedded 

 in its mother's flesh. The embryo must start on this 

 cannibal existence almost immediately after it is 

 fertilized. In fact, the connection between the 

 embryo and its mother is so highly developed and 

 specialized in Man that no further evolution along 

 these lines seems conceivable. 



It is often the failure of such a specialized organ to 

 be able to evolve to meet new conditions which leads 

 to the extinction of a species, and some authors have 

 suggested that Man may eventually die out because 

 of the over-specialization of his placenta. 



Growth 



Many animals never stop developing their whole 

 life long. It is only for convenience that we say that 

 a man becomes adult at the age of twenty-one. And 

 similarly with other animals, there is often no point at 

 which we can say definitely that the period of change 

 is over. This is particularly the case with animals 

 which go on growing throughout their whole 

 existence. With others, such as butterflies, once the 

 pupa has metamorphosed into a butterfly, no more 

 change is possible, and there can be no objection to 

 calling the animal adult. All animals which grow, 

 change not only in absolute size, but also in pro- 

 portions and therefore in shape. This alteration is 

 most rapid, of course, in the early years of life and 



