It is undeniable that strong impressions upon the sense of sight, affecting the parent during conception or an early stage of pregnancy, may and do influence the formation of the embryo, and it has consequently been asserted that the sight of the eggs lying in the nest has such an influence on the hen Cuckoo, that her egg, which is ready to be laid, assumes the colour and markings of those before her. This is not, however, supported by facts, for the egg of a Cuckoo is frequently found with eggs which do not in the least resemble it (_e.g._ those of the Hedge-Sparrow); or with eggs which, from the nature of the nest, could not have been seen by the Cuckoo (as in the case of the Redstart, Wren, or Willow Wren); or deposited in a nest before a single egg had been laid therein by the rightful owner. Again, two Cuckoo’s eggs of a different colour have been found in the same nest. If both were laid by one bird, we have a proof that the same Cuckoo does not always lay eggs of the same colour; if laid by different birds, then the Cuckoo is not so impressionable as has been supposed. What really takes place, I believe, is this:—The Cuckoo lays her egg upon the ground; the colour of the egg is variable according to the condition of the ovary, which depends upon the age of the bird, the nature of its food, and state of health at the time of oviposition. With her egg in her bill, the bird then seeks a nest wherein to place it. I am not unwilling to accept the suggestion that, being cognizant of colour, she prefers a nest which contains eggs similar to her own, in order that the latter may be less easily discovered by the foster parents. At the same time the egg in question is so frequently found amongst others which differ totally from it in colour, that I cannot think the Cuckoo is so particular in her choice as Dr. Baldamus would have us believe. The manner in which “the cuckowe’s bird useth the sparrow,” “oppressing his nest,” living upon him, and finally turning him adrift, has furnished a theme for poets and prose writers in all ages, and has awakened in no small degree the speculative powers of naturalists. The story is as old as the hills, and it would probably be difficult, if not impossible, to trace it to its origin. It was known to the ancients that the Cuckoo leaves its eggs to be hatched by other birds, but they mingled fact with fable, believing, or at all events asserting, that the young Cuckoo devoured not only its foster brothers and sisters, but ultimately its foster parents. Hence the expression which Shakespeare put into the mouth of the Earl of Worcester to the effect that the youngster “Grew by our feeding to so great a bulk That even our love durst not come near his sight For fear of swallowing.”—_Henry IV._ act v. sc. 1.