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 saxatilis 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 as 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 larvæ, 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-
