Page:The Harveian oration, 1873.djvu/35

 29 — I do not say definitely of precisely the same morphological importance — has often been figured as existing in the hearts of most or all Reptilia below Crocodilina ; and it serves in them to close up and expel the blood from the pulmonary compartment of their imperfectly divided ventricle. Such being the function of this moderator band, what is its morphological bearing, and what traces can we find of it in ourselves, tempting us to speculate as to the nature of the secret bond which brings us into rela- tions of affinity not only to the mammalian class, but with an older stock, the many- sided potentialities of which embraced not only mammals, but all warm-blooded animals, and not only all warm-blooded creatures, but warm-blooded animals and reptiles also? The valves of the heart in the higher vertebrata, when regarded from this point of view of development — the safest if not the sole cri- terion of homology — may be spoken of as being but trabeculae flaked off from the inner surface of the wall of a muscular sac, and subsequently made more or less membran- ous in the way of specialisation and its