As development progresses four small pads make their appearance; two of these lie on either side of the body back of the head and the other two arise near the posterior end.  They are far from being wings and legs, but as day follows day they become molded into somewhat similar limbs, as much alike in general plan as the four legs of a lizard; subsequently the ones at the front change into real wings and the hinder ones become legs.  Meanwhile the internal organs slowly transform from fishlike structures into things that display the characteristics of reptilian counterparts, and only later do they become truly avian.  Last of all the finishing touches are made, and the whole creature becomes a particular kind of a bird which picks its way out of the shell and shifts for itself as a chick.

Only a few of the countless details have been mentioned which demonstrate the resemblance of the successive stages first to fishes, and later to amphibia and reptiles.  We have a wide choice of materials, but even the foregoing brief list of illustrations shows that the order in which the stages follow is the one which comparative anatomy independently proves to be the order of the evolution of fishes, amphibia, reptiles, and birds.  Why, now, should it be necessary for a developing bird to follow this order?  The answer has been found in the immense array of embryological facts that investigators have verified and classified, that all tell the same story.  It is, that birds have arisen by evolution from ancestors which were really as simple as the members of these lower classes.  It seems then that the only way a bird of to-day can become itself is to traverse the path along which its progenitors had progressed in evolution.  Stating its conclusions precisely, science formulates the principle in the following words:  individual development is a brief resume of the history of the species in past times, or, more technically, ontogeny recapitulates phylogeny.  To be sure, the full history is not reviewed in detail, for the chick embryo does not actually swim in water and breathe by means of gills.  Only a condensed account of evolution of its kind is presented by an embryo during its development; as Huxley and Haeckel have put it, whole lines and paragraphs and even pages are left out; many false passages of a later date are inserted as the result of peculiar larval and embryonic needs and adjustments.  But in its major statements and as a general outline, the account is a trustworthy natural document submitted as evidence that higher species of to-day have evolved from ancestors which must have been like some of the present lower animals.