xvin ORGANISATION AND EVOLUTION 421 



Bearing this in mind, we find much significance in the 

 enormous differences in fertility that are to be found in 

 different divisions of the organic world. This fertility 

 seems to be at its highest where individual organisation is 

 lowest, that is to say, among Protozoa. It is at its lowest 

 in the human race. 1 It is calculated that a single Para- 

 mcecium would, if unchecked, produce two hundred and 

 sixty-eight billions of descendants in one month, while 

 another Infusorian is credited with a possible progeny of 

 a hundred and seventy billions in four days. 2 The birth 

 and death roll of many parasites is small in comparison 

 with these figures, but it has been computed that 

 60,000,000 eggs are contained in the body of a single 

 Nematode, 3 while, to pass to insects, Huxley calculated 

 that the helpless Aphis, the prey of countless insects, is so 

 prolific that in ten generations the produce of one indivi- 

 dual would contain more substance than 500,000,000 men. 4 

 Speaking generally, it appears that Invertebrates are more 

 prolific than Vertebrates, excepting fish, 5 while among 

 Vertebrates we find on the whole a regular descent from 

 Fish, through Amphibia and Reptiles to Birds and 

 Mammals. This is brought out in the following figures 

 which I take from Mr. Sutherland's valuable work. 6 



Number of young produced per annum by each female. 



Fish 646,000 



Amphibia (average of 20 species) 441 



Reptiles (average of 39 species) 17 



Birds (average of 2000 species) 5 



Mammals (average of 82 species) 3-2 



Higher orders of Mammals 1-3 



Ape and Man 



In interpreting these figures, we must bear in mind that 

 we are not always comparing like with like. In the case 



per adult, not per annum. A comparatively slow breeding animal may in 

 this sense have a high birth rate if it is long lived. (See Weismann, 

 Essays in Heredity, '* On the Duration of Life.") 



1 The Elephant is perhaps less fertile than man, but the Elephant is 

 dying out, while the human race is increasing in numbers. 



2 Ray Lankester, Comparative Longevity, p. 74. 



3 Cambridge Natural History, Vol. II. p. 162. 



4 Op. cit. Vol. VI. p. 589. 5 Ray Lankester, loc. cit. 

 6 Op. cit. p. 41. 



