COURTING DISPLAY INSTINCTS IN BIRDS. 405 



uncertain. In 9 species he assists her in nidification, but not in 

 incubation. In 56 species he takes part in incubation, but it is 

 uncertain whether he does so also in nidification ; and in 43 

 species he helps to incubate only. In 28 species the male is 

 stated neither to build nor to incubate, and as 22 of these belong 

 to either the Duck or Pheasant families, whilst the Buff and 

 Great Bustard are included in the other 6 — all great displayers — 

 we can understand this on the principles already adverted to. 

 Also, through the law of reversion, but not very easily otherwise, 

 we can understand why the male Pheasant (as the best known) 

 has sometimes been known to incubate. 



There are thus left 39 species where the participation of the 

 male either in one'way or the other, is doubtful, but as more 

 than half of these are more or less rare birds, and the greater 

 number of shy, retiring, or nocturnal habits, it is obvious that 

 a fair proportion of them (since there is not the precedent reason 

 against it) will in time be added to the affirmative list. 



Let us, however, take certainties only, or what, within a 

 moderate margin, may be assumed to be such. There are 173 

 species in which the part played by both sexes, in the matters 

 under consideration, is given as ascertained, and in 145 of these 

 the male assists the female either in incubation or nidification, 

 or both. That is a proportion of more than five-sixths. Where 

 the assistance is in nidification only, the number given is 85 — 

 practically half — and where it is confined to incubation, 123, 

 which is nearly three-quarters. The supposed exceptions, there- 

 fore, outnumber the supposed rule. How is this to be accounted 

 for ? Have large numbers of male birds acquired certain domestic 

 habits which did not originally and naturally belong to them, 

 but only to the female ? Or were such habits — once common to 

 both the sexes — lost by many males, in the course of evolution, 

 owing, a3 we may surmise, to the advantages derived from 

 division of labour, differentiation, and the economy of force — 

 Malpertuis' law of " least action? " If the instincts which give 

 rise to these habits have, as I hold, been evolved out of the 

 primary sexual one, then, as this came first and is universal, 

 the second of these suggested explanations would seem to be 

 the more probable. What is the direct evidence ? I have, at 

 various times, observed and recorded certain facts in the nuptial 



