After thus seeing that even in the developed fish with its complex locomotive apparatus, this law of the transverse strain holds in a qualified way, we shall understand how much more it must hold in any form that may be supposed to initiate the vertebrate type--a form devoid of that segmentation by which the vertebrate type is more or less characterized. We shall see that assuming a rudimentary animal, still simpler than the _Amphioxus_, to have a feeble power of moving itself through the water by the undulations of its body, or some part of its body, there will necessarily come into play certain reactions which must affect the median portion of the undulating mass in a way unlike that in which they affect its lateral portions. And if there exists in this median portion a tissue which keeps its place with any constancy, we may expect that the differential conditions produced in it by the transverse strain, will initiate a differentiation. It is true that the distribution of the viscera in the _Amphioxus_, Fig. 191, and in the type from which we may suppose it to have arisen, is such as to interfere with this process. It is also true that the actions and reactions described would not of themselves give to the median portion a cylindrical shape, like that of the cartilaginous rod running along the back of the _Amphioxus_. But what we have here to note in the first place is, that these habitual alternate flexions have a tendency to mark off from the outer parts an unlike inner part, which may be seized hold of, maintained, and further modified, by natural selection, should any advantage thereby result. And we have to note in the second place, that an advantage _is_ likely to result. The contractions cannot be effective in producing undulations, unless the general shape of the body is maintained. External muscular fibres unopposed by an internal resistant mass, would cause collapse of the body. To meet the requirements there must be a means of maintaining longitudinal rigidity without preventing bends from side to side; and such a means is presented by a structure initiated as described. In brief, whether we have or have not the actual cause, we have here at any rate “a true cause.” Though there are difficulties in tracing out the process in a definite way, it may at least be said that the mechanical genesis of this rudimentary vertebrate axis is quite conceivable. And even the difficulties may, I think, be more fully met than at first sight seems possible. [Illustration: Fig. 191.]