This case is striking, partly because it is a very favorable one, but mainly because it has been so carefully studied. There can be no doubt that equally careful study would reveal the same transition in many other cases. Nor are such transitions confined to the lower forms of life, though they are probably more abundant there. According to Cope, the nicest gradations may be traced between some of the extinct mammalian species so abundant in the Tertiary deposits of the West--especially between the species of the extinct generalized family of _Oredontidæ_[36] The same is probably true of the many extinct species of the horse family. It is interesting to observe that the details of the process of change in the forms of _Planorbis_ are in accord with Dr. Romanes’s views. The change does not seem to have been uniform but somewhat paroxysmal. The forms seem to remain stable for a long time, and then a few break into several different forms, while the more rigid die out. It is as if cross-breeding had kept the type true, but at the same time increased its tendency to variation, until finally one or more varieties became sexually isolated and thus formed new species. 5. But still the question remains: Why are transitional forms _rare_ in all cases, especially between species--so rare that they are eagerly sought and highly prized? I believe that the true reason of this is that _the steps of evolution are not always uniform_. Nearly all evolutionists have assumed and even insisted on uniformity, as the opposite of catastrophism and of supernaturalism, and therefore as essential to the idea of evolution. They say that the constancy of the action of the forces of change necessitates the uniformity of the rate of change. But, in fact, this is not always nor even usually true. Causes or forces are constant, but phenomena everywhere and in every department of Nature are _paroxysmal_. The forces producing storms and lightning, and volcanoes and earthquakes, are or may be constant; yet the phenomena are in the highest degree paroxysmal. Wherever in nature we have a constant force and a strong resistance, we find more or less paroxysmal action. For this reason the wind blows in puffs, the friction of wind on water produces waves, water running in small pipes issues in pulses. The reason is obvious, as may be seen by the following examples: Suppose lifting forces within the earth are resisted by crust-rigidity. The forces accumulate uniformly until the resistance gives way, and suddenly we have an earthquake. Water running with great resistance in small pipes is checked, but soon accumulates additional force, which overcomes the resistance, only to be again checked, and so on, and therefore runs in pulses. Now, the course of evolution of the whole earth may be likened to such a current; there are forces of movement and forces of resistance--progressive forces and conservative forces. The progressive force is accumulative, the resisting force is constant.