VARIATION IN CUCKOOS' EGGS. 177 



Quite irrespectively of this, how would it be possible to explain, by 

 means of the theory of selection, the fact that there are a great number of 

 Cuckoos' eggs which have a particular type of colouring not to be found in 

 any eggs known to us, and others marked like eggs with which eggs of the 

 Cuckoo are seldom placed. We must therefore cast about for another 

 explanation. In a number of species of birds we see that the eggs differ 

 considerably in colour and marks when they come from places far apart. To 

 quote a few examples : eggs of Phylloscopus trochilus from Lapland are, 

 contrary to those found in our parts, marked with dark spots, so dark as 

 almost to be mistaken for eggs of Phylloscopus rufus. Again, whilst 

 spotted eggs of the Redstart are rare here, examples are frequent in high 

 northern latitudes; and whereas Caccabis saxatills lays distinctly spotted 

 eggs in the alpine regions, its eggs from Greece are monochromous, or but 

 very slightly marked. 



Now, as Wickmann has demonstrated that eggs take their colour from 

 the transposing products of the blood, so must we lead back the varieties 

 of colouring to the variety of these transposing products, and the latter 

 again to the chemical or physical properties of the blood. We must look 

 upon food aa the chief cause of the difference in the formation of the blood, 

 for according to its different chemical properties it will produce lesser or 

 greater variety in the composition of the blood. We must therefore take, 

 as the cause of the variation in the colouring of the eggs of the same bird 

 from different places, the difference of food according to the place of their 

 residence. Not that different nourishment would produce an immediate 

 change in the colour of the eggs — for we know that every female bird will, 

 during its whole life, unless pathological changes should occur, lay the 

 same, or at least very similarly, coloured eggs — but the difference in food 

 will, in the young female bird, whilst the body is developing, have an abiding 

 influence upon its blood-forming organs, and determine the colour of her 

 future eggs. It is clear that apparently similar food can produce different 

 results, for we often see that insects and larvae, externally alike, have, 

 chemically, quite different bodies ; and, again, quite distinct insects are 

 chemically alike. 



If, on the one hand, the variation in the eggs of different female birds 

 of the same species is occasioned in this way, the law of heritage confines 

 it on the other. We see that Shrikes and Pipits lay very different eggs, 

 but notwithstanding the number of varieties there is a decided type running 

 through them all. Here we see a certain inherited resemblance, whereas 

 in other cases the eggs are so completely distinctive as to be unrecognizable. 

 If we apply this to the Cuckoo, we are not astonished if almost every bird 

 lays differently coloured eggs, because the difference of food arising from 

 the various foster-parents, according to their kind and individuality, pro- 



Zool. 4th ser. vol. III., April, 1899. N 



