A student who relied on Professor Haeckel's description, would obtain an entirely erroneous idea of the development of the human embryo.... It is a matter for great regret that a book of 900 pages, bearing such a title, should be allowed to appear, in which the account of the actual development of the human embryo is so inadequate or even erroneous. Far more fundamental, however, is a remark of Mr. Mivart's, that if, as Darwinians say, the development of the individual is an epitome of that of the species, the latter must like the former be due to the action of definite innate laws unconsciously carrying out definite preordained ends and purposes. For although cells or embryos may be indistinguishable from one another, and may appear to us identical in constitution, their differences are absolute. Each is determined to be one sort of animal and no other, and can live at all only on condition of developing towards the prescribed form.--Therefore, whatever evidence the embryonic forms may be supposed to afford in support of Evolution, they have nothing in common with the haphazard process of Natural Selection. And here again Professor Huxley found himself obliged to enter his _caveat_, and to intimate his opinion that some of his friends were inclined to build too confidently upon this foundation. As his biographer Professor Weldon writes in the _Dictionary of National Biography_: Darwin had suggested an interpretation of the facts of embryology which led to the hope that a fuller knowledge of development might reveal the history of all the great groups of animals at least in its main outlines. This hope was of service as a stimulus to research, but the attempt to interpret the phenomena observed led to speculations which were often fanciful and always incapable of verification. Huxley was keenly sensible of the danger attending the use of a hypothetical explanation, leading to conclusions which cannot be experimentally tested, and he carefully avoided it.... In the preface to the _Manual of the Comparative Anatomy of Invertebrated Animals_, he says: "I have abstained from discussing questions of ætiology,[233] not because I underestimate their importance, or am insensible to the interest of the great problem of Evolution, but because, to my mind, the growing tendency to mix up ætiological speculations with morphological generalizations will, if unchecked, throw Biology into confusion."