Now to both Science and Christianity, and especially to Science, this enrichment of Evolution is important. And, on the part of Christianity, the contribution to the system of Nature of a second barrier is of real scientific value. At first it may seem merely to increase the difficulty. But in reality it abolishes it. However paradoxical it seems, it is nevertheless the case that two barriers are more easy to understand than one--two mysteries are less mysterious than a single mystery. For it requires two to constitute a harmony. One by itself is a Catastrophe. But, just as the recurrence of an eclipse at different periods makes an eclipse no breach of Continuity; just as the fact that the astronomical conditions necessary to cause a Glacial Period will in the remote future again be fulfilled constitutes the Great Ice Age a normal phenomenon; so the recurrence of two periods associated with special phenomena of Life, the second higher, and by the law necessarily higher, is no violation of the principle of Evolution. Thus even in the matter of adding a second to the one barrier of Nature, the Third Kingdom may already claim to complement the Science of the Second. The overthrow of Spontaneous Generation has left a break in Continuity which continues to put Science to confusion. Alone, it is as abnormal and perplexing to the intellect as the first eclipse. But if the Spiritual Kingdom can supply Science with a companion-phenomenon, the most exceptional thing in the scientific sphere falls within the domain of Law. This, however, is no more than might be expected from a Third Kingdom. True to its place as the highest of the Kingdoms, it ought to embrace all that lies beneath and give to the First and Second their final explanation.
How much more in the under-Kingdoms might be explained or illuminated upon this principle, however tempting might be the inquiry, we cannot turn aside to ask. But the rank of the Third Kingdom in the order of Evolution implies that it holds the key to much that is obscure in the world around--much that, apart from it, must always remain obscure. A single obvious instance will serve to illustrate the fertility of the method. What has this Kingdom to contribute to Science with regard to the Problem of the origin of Life itself? Taking this as an isolated phenomenon, neither the Second Kingdom, nor the Third apart from revelation, has anything to pronounce. But when we observe the companion-phenomenon in the higher Kingdom, the question is simplified. It will be disputed by none that the source of Life in the Spiritual World is God. And as the same Law of Biogenesis prevails in both spheres, we may reason from the higher to the lower and affirm it to be at least likely that the origin of life there has been the same.
There remains yet one other objection of a somewhat different order, and which is only referred to because it is certain to be raised by those who fail to appreciate the distinctions of Biology. Those whose sympathies are rather with Philosophy than with Science may incline to dispute the allocation of so high an organism as man to the merely vegetal and animal Kingdom. Recognizing the immense moral and intellectual distinctions between him and even the highest animal, they would introduce a third barrier between man and animal--a barrier even greater than that between the Inorganic and the Organic. Now, no science can be blind to these distinctions. The only question is whether they are of such a kind as to make it necessary to classify man in a separate Kingdom. And to this the answer of Science is in the negative. Modern Science knows only two Kingdoms--the Inorganic and the Organic. A barrier between man and animal there may be, but it is a different barrier from that which separates Inorganic from Organic. But even were this to be denied, and in spite of all science it will be denied, it would make no difference as regards the general question. It would merely interpose another Kingdom between the Organic and the Spiritual, the other relations remaining as before. Any one, therefore, with a theory to support as to the exceptional creation of the Human Race will find the present classification elastic enough for his purpose. Philosophy, of course, may propose another arrangement of the Kingdoms if it chooses. It is only contended that this is the order demanded by Biology. To add another Kingdom mid-way between the Organic and the Spiritual, could that be justified at any future time on scientific grounds, would be a mere question of further detail.
Studies in Classification, beginning with considerations of quality, usually end with a reference to quantity. And though one would willingly terminate the inquiry on the threshold of such a subject, the example of Revelation not less than the analogies of Nature press for at least a general statement.