Now, although the evolution hypothesis supposes that, starting from an ancestral stock, the modifications of structure have been produced in offspring descended from parents of that same stock, which have transmitted acquired peculiarities to their immediate progeny, and so on indefinitely, yet there must have been a time when the diverging species became distinct and peculiar organisms, and when it became impossible for any crossing of these organisms to take place. All the supposed modifications, therefore, have taken place within the limits of an actual descent of one kind of animal from another, each successive pair belonging to the species from which they were individually generated. In this descent of lives from lives, there came about changes which in progress of time led to two animals as wide asunder as the man and the ostrich, or as the man and the horse, and yet the causes which were powerful enough to produce these widely diverging species were not powerful enough to break up all unity of plan in some one or more respects. If naturalists of the evolution school would explain how there has come to be, for example, in the skeleton of the _vertebrata_, a bony structure called the spine, in which a certain resemblance and a certain function obtain throughout the whole class, and yet one species creeps upon its belly, another walks on four legs, and another on two, and one flies in the air and another never can do so, and how this could be without any design or special interposition of a creating power, but that the whole of this uniformity amid such diversity has arisen from acquired habits among the different descendants from an aboriginal stock that had no such habits in either mode of locomotion, and no organs for such modes of life, they would at least be able to commend their theory to a better appreciation of its claims than is now possible to those who want "grounds more relative" than a naked hypothesis. 3. The argument from embryology requires for its appreciation a careful statement of its abstract proposition, and a statement of it in a concrete form. As an abstract proposition, embryology, or the comparison of the development of different organisms under their embryonic stages, shows that in the earliest stage of any organism it has the greatest number of characters in common with all other organisms in their earliest stage; that at a later stage its structure is like the structures displayed at corresponding phases by a less extensive number of organisms; that at each subsequent stage the developing embryo becomes more and more distinguished from the groups of embryos that it previously resembled; and that this divergence goes on, until we reach the species of which the embryo is a member, in which the class of similar forms is finally narrowed to that species.