284 THE BIOLOGY OF BIRDS 



temporary variations in the direction of increased parental 

 care. The divergences in the number of eggs in the nests 

 of birds are not interpretable in terms of individual income 

 and expenditure ; they have been slowly wrought out in the 

 course of ages in relation to the broad issues of life. 



§ 12. Sterility 



The significance of sterility is very obscure, and we cannot 

 do much more than indicate the various phenomena to which 

 the term is applied : (a) somatic sterility, where the defect 

 is not a fundamental inability to produce germ-cells ; (b) 

 modificational sterility, where the reproductive organs cease 

 to produce germ-cells because of some deteriorative influence ; 

 {c) variational sterility, where the reproductive organs never 

 produce germ-cells, or eff"ective germ-cells, although all the 

 nurtural conditions are normal ; and {d) gametic sterility, 

 where the germ-cells are incompatible. 



Somatic Sterility. — The researches of Maynie R. Curtis 

 and Raymond Pearl (19 15) make it possible to distinguish 

 " somatic sterility " in fowls (due to some obstruction or 

 disturbance) from constitutional or variational sterility due 

 to some defect in the inheritance. Somatic sterility may be 

 caused by some obstruction in the oviduct or a general 

 lowering of the physiological tone of the individual so that 

 no yolk is formed. In some cases it is impossible for the 

 ovum to enter the oviduct ; it has to pass into the body- 

 cavity, where it is absorbed. An interesting detail is that 

 if there be a stoppage in the oviduct at any level, the tracts 

 on each side of the block pass through the normal cyclic 

 changes, co-ordinated with cyclic changes in the ovary. 



Modificational Sterility. — A cessation in the production 

 of germ-cells may be brought about by some drastic change 

 of diet, such as feeding fowls on flesh ; or by some poisoning ; 

 or by some subtle changes such as are involved in confinement. 

 Obviously the absence of wiUingness to pair, often exhibited 

 by captive birds, must be distinguished from something 

 deeper, namely a stoppage of germ-cell formation. 



