62 THE EVOLUTION OF MAN 



most striking characteristics, in contrast to the lower animals, 

 are its helplessness and educability. Its helplessness has un- 

 doubtedly been an important factor in the development of the 

 family, while the long period of growth before maturity is 

 reached has exerted a great influence in the development of 

 man's psychic powers, which distinguish him so markedly from 

 all other animals. 



Statistics of various animals show that the proportion of 

 male and female births is approximately equal; the ratio in 

 man being about 106 males to 100 females. Multiple births 

 are the usual rule in most of the lower mammals but not in the 

 primates. Occasionally, however, it occurs and in man even 

 sextuplets have been recorded. There have been some 500 

 theories proposed as to the cause of sex determination and 

 as to methods for controlling it, but the accumulated evidence 

 of the last few years clearly shows that sex is determined at 

 the time of fertilization, just as are all somatic characters, 

 and that it cannot later be changed by any means. Inasmuch 

 as the ossification of the bones of a female embryo precedes 

 slightly that of the male embryo, a radiograph may perhaps 

 aid in ascertaining the sex of the child before birth. 



If we summarize the facts regarding man's early develop- 

 ment, which are given above, it is evident that he starts his 

 development as a unicellular animal, becomes a metazoan of 

 the invertebrate type, develops the structure of a vertebrate, 

 and ultimately becomes a mammal. Many of the structures 

 of his body pass through temporary stages which are typical of 

 the adult condition in various classes of the vertebrates and, 

 in general, his organs are developed in a manner similar to 

 that in other animals. Man's development, however, parallels 

 that of the anthropoid apes for a longer time than that of 

 other mammals and at birth the child has ape-like characters 

 that later disappear. These facts of human development 

 associate man closely with the anthropoid apes and give evi- 



