INSTINCT 215 



If the two were evolved pari passu, then there 

 must have been some necessary relation between their 

 growth. That the necessity in the egg for the in- 

 stinct in the bird could not have produced the instinct 

 is evident. That the instinct in the bird could have 

 so modified the egg as to render incubation necessary, 

 1 see no reason to believe. It may, of course, be 

 claimed that a bird was hatched with a slight dispo- 

 sition to incubate, and that it laid an egg with a small 

 necessity for being incubated — both at the same time 

 by chance — and that these two things increased 

 through many generations. This is mere assumption. 



The instinct to incubate is of extreme antiquity, as 

 is shown by the fact that it is almost universal among 

 birds. The original avian stock must have incubated, 

 otherwise the existence of the instinct among birds 

 now living could not be universal. This shows that 

 the instinct is not transient, and it implies that it 

 could not have been suddenly acquired by evolution. 

 While the birds themselves have changed greatly in 

 their structure, yet the instinct to incubate has sur- 

 vived. 



In this case, then, we have an extremely long 

 record, extending from the ancestors of Archseopteryx, 

 which began somewhere in the Paleozoic, during 

 which the instinct to incubate has persisted in the 

 birds. An instinct so enduring could not have sprung 

 up suddenly from habit or by natural selection. If it 

 was evolved it must of necessity have originated at 

 the time when the cold-blooded reptile with a three- 

 chambered heart and a sluggish circulation of blood 

 had been changed by natural selection into a warm- 

 blooded, feathered bird, with a four-chambered heart 

 and with a much more active circulation of the blood. 

 The change in the temperature of the evolved bird, as 



