cuckoos' eggs and evolution. 279 



Cuckoos' eggs have attained similarity to those of their fosterers 

 but really this evolution has been brought about in the simplest 

 manner. We may presume that at one time all the eggs of any 

 one particular kind of Cuckoo were vi^hite and that they were 

 laid promiscuously in any birds' nests which were handy. If 

 among these there had been a suitable fosterer with white eggs, 

 there would have been little or no evolution or alteration in the 

 Cuckoo's egg. What would have occurred would merely have 

 been that those white eggs placed in fosterers' nests with white 

 eggs would have been hatched in far greater numbers than those 

 which were placed with contrasting eggs and gradually the latter 

 would have been eliminated whilst the former would have sur- 

 vived. This is, indeed, what seems to have happened in a few 

 cases as I will show later on. But in the majority of instances 

 it is evident that there were either no suitable fosterers laying 

 white eggs or that they were not in sufficient numbers to be able 

 to act both as foster-parents to the Cuckoo and also perpetuate 

 their own race. Amongst the Cuckoos' eggs, however, there 

 were probably some which were abnormally coloured with faint 

 blue, red, or some other colour. Now, if these were placed 

 alongside the eggs of foster birds with somewhat similarly 

 coloured eggs, they would escape detection in greater proportion 

 than the quite pure white ones, whilst among the former again 

 those eggs with the deepest tinge v/ould escape more often than 

 the others and so, just by the gTadual elimination of the most 

 unfit, we finally obtain a coloui'ed egg so close to that of the 

 fosterer that no further elimination is required. 



Evolution in the Cvickoo's egg has, in fact, proceeded as in all 

 other phases of evolution. Destruction and creation are synony- 

 mous terms in evolution ; the unfit have perished and the more 

 fit survived, the latter in their turn being wiped out as yet 

 better adapted individuals took their place. 



In discussing the three degrees of evolution, referred to above, 

 it will pi'obably be more convenient if we examine the third 

 degree, i. e., that of perfect evolution first, and bring forward 

 whatever proofs are possible in support of my theories. 



Perfect evolution in a Cuckoo's egg must mean that a degree 

 of resemblance in colour, combined with size and shape, has been 

 arrived at which is sufficient to ensure the continuation of the 

 species in its proper numerical proportion with, or without, other 

 controlling factors. There will, therefore, be no variation in 

 such egg and comparatively little variation in the genera and 

 species of birds selected as fosterers. Finally, such a state of 

 evolution also infers that there is an ample supply of suitable 

 fosterers laying eggs of the type to resemble which the Cuckoo's 

 egg has been evolved. 



No finer examples can be given of perfected evolution than the 

 eggs of Clamator jacohinus, the Pied Crested Cuckoo, and Eiero- 

 coccyx vcirius, the Common Hawk Cuckoo, the two most common 



