xvin ORGANISATION AND EVOLUTION 425 



nest-builders. 1 Summing up on the fertility of fish, Mr. 

 Sutherland says : 



" Of species that exhibit no sort of parental care, the average 

 of forty-nine give 1,040,000 eggs to a female each year ; while 

 among those which make nests or any apology for nests the 

 number is only about 10,000. Among those which have any 

 protective tricks, such as carrying the eggs in pouches or attached 

 to the body, or in the mouth, the average number is under 1,000 ; 

 while among those whose care takes the form of a uterine or 

 quasi-uterine gestation which brings the young into the world 

 alive, an average of fifty-six eggs is quite sufficient." 2 



Now, the mere postponement of birth is of course a 

 purely physical development. Still, it is an important 

 step in the organisation of life and in curtailment of the 

 struggle for existence. The higher development of corre- 

 lation between separate lives begins with parental care in 

 the ordinary sense. Its efficacy is best seen in the higher 

 Classes. For example, Mr. Sutherland 3 divides Birds into 

 three grades, his account of which 4 may be summed up in 

 tabular form : 



Lowest grade. 

 The young are 



capable. 

 There is no nest. 



Average number 

 of eggs 12-5 per 

 annum. 



Middle grade. 

 Young dependent but 



can run at birth. 

 Loose nest. 



Average 7*66. 



Highest grade. 

 Young callow. 



High skill in nest 

 building or in find- 

 ing substitutes. 



Average 4-5. 



Here it will be seen that with the development of 

 intelligence and of parental care, though the young are 

 more callow, fewer of them are needed to maintain the 

 numbers of the species. 



With more complication of detail, Mr. Sutherland 

 shows that the same law holds among Mammals. Thus 

 from the lowest organisms up to the highest, culminating 



1 We must except the most efficient nest-builders, such as the stickle- 

 back, who hatches from 20 to 90 eggs only at a time (Sutherland, p. 36.) 



2 Op. tit. Vol. I. p. 40. 3 Op. tit. p. 58. 

 4 Op. tit. pp. 54-71. 



