The Darwinian theory of "Natural Selection" shortly stated thus: 1

may be

Every kind of animal and plant tends to increase in numbers in a geometrical progression.

Every kind of animal and plant transmits a general likeness, with individual differences, to its offspring. Every individual may present minute variations of any kind and in any direction.

Past time has been practically infinite.

Every individual has to maintain a very severe struggle for existence, owing to the tendency to geometrical increase of all kinds of animals and plants, while (from the constancy of physical conditions acting as a continual check on such increase) the total amount of animal and vegetable population (man and his agency excepted) remains almost stationary from year to year.

Thus, every variation of a sort tending to save the life of the individual possessing it, or to enable it more surely to propagate its kind, will in the long run be preserved, and the organism that has it will transmit its favourable peculiarity to some of its offspring, which peculiarity will thus become intensified till it reaches the maximum degree of utility. On the other hand, individuals presenting unfavourable peculiarities will be ruthlessly destroyed. The action of this law of "Natural Selection" may thus be well represented by the convenient expression " survival of the fittest." "12

Now this conception of Mr. Darwin is perhaps the

1 See Mr. Wallace's recent work, entitled "Contributions to the Theory of Natural Selection," where, at p. 302, it is very well and shortly stated.

2 "Natural Selection" is happily so termed by Mr. Herbert Spencer in his "Principles of Biology."

most interesting theory, in relation to natural science, which has been promulgated during the present century. Remarkable indeed is the way in which it groups together so vast and varied a series of biological1 facts, and even paradoxes, which it appears more or less clearly to explain, as the following instances will show.

By this theory of "Natural Selection," light is thrown on the more singular facts relating to the geographical distribution of animals and plants; for example, on the resemblance between the past and present inhabitants of different parts of the earth's surface. Thus in Australia remains have been found of creatures closely allied to kangaroos and other kinds of pouched beasts, which in the present day exist nowhere but in the Australian region. Similarly in South America, and nowhere else, are found sloths and armadillos, and in that same part of the world have been discovered bones of animals different indeed from existing sloths and armadillos, but much more nearly related to them than to any other kinds whatever. Such coincidences between the existing and antecedent geographical distribution of forms are numerous. Again, "Natural Selection" serves to explain the circumstance that often in adjacent islands we find animals closely resembling, and appearing to represent, each other; while if certain of these islands show signs (by depth of surrounding sea or what not) of more ancient separation, the animals inhabiting them exhibit a corresponding divergence.2 The explanation consists in representing the forms inhabiting the islands as being the modified

1 Biology is the science of life. It contains zoology, or the science of animals, and botany, or that of plants.

2 For very interesting examples, see Mr. Wallace's "Malay Archipelago."

descendants of a common stock, the modification being greatest where the separation has been the most prolonged.

"Rudimentary structures" also receive an explanation by means of this theory. These structures are parts which are apparently functionless and useless where they occur, but which represent similar parts of large size and functional importance in other animals. Examples of such "rudimentary structures" are the foetal teeth of whales, and those of the front part of the upper jaw of ruminating quadrupeds. These foetal structures are minute in size, and never cut the gum, but are re-absorbed without ever coming into use, while no other teeth succeed or represent them in the adult condition of those animals. The mammary glands of all male beasts constitute another example, as also does the wing of the apteryx-a New Zealand bird utterly incapable of flight, and with the wing in a quite rudimentary condition (whence the name of the animal). Yet this rudimentary wing contains bones which are miniature representatives of the ordinary wing-bones of birds of flight. Now, the presence of these useless bones and teeth is explained if they may be considered as actually being the inherited diminished representatives of parts of large size and functional importance in the remote ancestors of these various animals.

Again, the singular facts of homology are similarly capable of deeper explanation by "Natural Selection." Homology" is the name applied to the investigation of those resemblances which have so often been found to underlie superficial differences between animals of very different form and habit. Thus man, the horse, the whale, and the bat, all have the pectoral limb, whether it be the arm, or fore-leg, or paddle, or wing, formed on essentially

the same type, though the number and proportion of parts may more or less differ. Again, the butterfly and the shrimp, different as they are in appearance and mode of life, are yet constructed on one common plan, of which they constitute diverging manifestations. No a priori reason is conceivable why such similarities should be necessary, but they are readily explicable on the assumption of a genetic relationship and affinity between the animals in question, assuming, that is, that they are the modified descendants of some ancient form-their common ancestor.

That remarkable series of changes which animals undergo before they attain their adult condition, which is called their process of development, and during which they more or less closely resemble other animals during the early stages of the same process, has also great light thrown on it from the same source. The question as to the singularly complex resemblances borne by every adult animal and plant to a certain number of other animals and plants-resemblances by means of which the adopted zoological and botanical systems of classification have been possible-finds its solution through the same hypothesis, classification becoming the expression of a genealogical relationship. Finally, by this theory-and as yet by this alone-can any explanation be given of that extraordinary phenomenon which is metaphorically termed mimicry. Mimicry is a close and striking, yet superficial resemblance borne by some animal or plant to some other, perhaps very different, animal or plant. The 'walking leaf" (an insect belonging to the grasshopper and cricket order) is a well-known and conspicuous instance of the assumption by an animal of the appearance of a vegetable structure (see illustration on p. 40);

and the bee, fly, and spider orchids are familiar examples of a converse resemblance. Birds, butterflies, reptiles, and even fish, seem to bear in certain instances a similarly striking resemblance to other birds, butterflies, reptiles and fish, of altogether distinct kinds. The explanation of this matter which "Natural Selection" offers, as to animals, is that certain varieties of one kind have found exemption from persecution in consequence of an accidental resemblance which such varieties have exhibited to animals of another kind, or to plants; and that they were thus preserved, and the degree of resemblance was continually augmented in their descendants. As to plants, the explanation offered by this theory might perhaps be that varieties of plants which presented a certain superficial resemblance in their flowers to insects, have thereby attracted such insects, and have so been helped to propagate their kind, the visit of certain insects being useful or indispensable to the fertilization of many flowers.

We have thus a whole series of important facts which Natural Selection" helps us to understand and coordinate. And not only are all these diverse facts strung together, as it were, by the theory in question; not only does it explain the development of the complex instincts of the beaver, the cuckoo, the bee, and the ant, as also the dazzling brilliancy of the humming-bird, the glowing tail. and neck of the peacock, and the melody of the nightingale; the perfume of the rose and the violet, the brilliancy of the tulip and the sweetness of the nectar of flowers; not only does it help us to understand all these, but it also serves as a basis of future research and of inference from the known to the unknown, and guides the investigator to the discovery of new facts which, when