presented with incomparably greater force, clearness, patience, and knowledge than had ever been seen before; and, second, the exposition of the principle of natural selection as a mechanism which might have caused, and probably did cause, evolution. Huxley, as has been shewn, like many other anatomists, was ready for the general principle of evolution. In fact, so far as it concerned the great independent types which he believed to exist among animals, he was more than prepared for it. Let us take a single definite example of his position. In his work on the Medusæ, he had shewn how a large number of creatures, at first sight diverse, were really modifications of a single great type, and he used language which, now that all zoölogists accept evolution in the fullest way, requires no change to be understood: "What has now been advanced will, perhaps, be deemed evidence sufficient to demonstrate,--first, that the organs of these various families are traceable back to the same point in the way of development; or, secondly, when this cannot be done, that they are connected by natural gradations with organs which are so traceable; in which case, according to the principles advanced in 57, the various organs are homologous, and the families have a real affinity to one another and should form one group.... It appears, then, that these five families are by no means so distinct as has hitherto been supposed, but that they are members of one great group, organised upon one simple and uniform plan, and, even in their most complex and aberrant forms, reducible to the same type. And I may add, finally, that on this theory it is by no means difficult to account for the remarkable forms presented by the Medusæ in their young state. The Medusæ are the most perfect, the most individualised animals of the series, and it is only in accordance with what very generally obtains in the animal kingdom, if, in their early condition, they approximate towards the simplest forms of the group to which they belong."