4 THE EVOLUTION OF MAN. 



nearly a year and three quarters. In the smaller Mammals, 

 the duration of embryonic development is, on the contrary, 

 much shorter. The smallest Mammals, the Harvest Mice, 

 develop fully in 3 weeks ; Rabbits and Hares in 4 weeks ; 

 Rats and Marmots in 5 weeks ; the Dog in 9, the Pig in 17, 

 the Sheep in 21, and the Stag in 36 weeks. Development is 

 yet more rapid in Birds. The Chick, under normal con- 

 ditions of incubation, requires only 3 weeks, or just 21 days 

 for its full development. The Duck, on the other hand, 

 takes 25, the Turkey 27, the Peacock 31, the Swan 42, and 

 the New Holland Cassowary 65 days. The smallest of all 

 Birds, the Humming-bird, quits the egg after the twelfth day. 

 It is, therefore, evident that in Mammals and in Birds the 

 duration of development within the egg-membranes stands 

 in a definite relation to the size of body attained by each 

 vertebrate species. But the latter is not the sole determin- 

 ing cause of the former. There are many other circum- 

 stances which influence the duration of individual develop- 

 ment within the membranes of the egg. 126 



In all cases, however, the duration of the Ontogeny 

 appears infinitely brief when compared with the enormous, 

 the infinitely long period during which the Phylogeny, or 

 gradual development of the ancestral series, took place. 

 This period is not to be measured by years and centuries, 

 but by thousands and millions of years. Many millions of 

 years must indeed have elapsed while the most perfect 

 vertebrate organism, Man, gradually developed from the 

 primaeval one-celled ancestral organism. The opponents of 

 the development theory, who regard this gradual develop- 

 ment of Man from lower animal forms, and his original 

 descent from a one-celled primitive animal as incredible, 



