Because, if there is no proof of it, all that I have been telling you goes for nothing in accounting for the origin of species.Are natural causes competent to play the part of selection in perpetuating varieties? Here we labour under very great difficulties.In the last lecture I had occasion to point out to you the extreme difficulty of obtaining evidence even of the first origin of those varieties which we know to have occurred in domesticated animals.I told you, that almost always the origin of these varieties is overlooked, so that I could only produce two of three cases, as that of Gratio Kelleia and of the Ancon sheep.People forget, or do not take notice of them until they come to have a prominence; and if that is true of artificial cases, under our own eyes, and in animals in our own care, how much more difficult it must be to have at first hand good evidence of the origin of varieties in nature! Indeed, I do not know that it is possible by direct evidence to prove the origin of a variety in nature, or to prove selective breeding; but I will tell you what we can prove--and this comes to the same thing--that varieties exist in nature within the limits of species, and, what is more, that when a variety has come into existence in nature, there are natural causes and conditions, which are amply competent to play the part of a selective breeder; and although that is not quite the evidence that one would like to have--though it is not direct testimony--yet it is exceeding good and exceedingly powerful evidence in its way.
As to the first point, of varieties existing among natural species, Imight appeal to the universal experience of every naturalist, and of any person who has ever turned any attention at all to the characteristics of plants and animals in a state of nature; but I may as well take a few definite cases, and I will begin with Man himself.
I am one of those who believe that, at present, there is no evidence whatever for saying, that mankind sprang originally from any more than a single pair; I must say, that I cannot see any good ground whatever, or even any tenable sort of evidence, for believing that there is more than one species of Man.Nevertheless, as you know, just as there are numbers of varieties in animals, so there are remarkable varieties of men.I speak not merely of those broad and distinct variations which you see at a glance.Everybody, of course, knows the difference between a Negro and a white man, and can tell a Chinaman from an Englishman.They each have peculiar characteristics of colour and physiognomy; but you must recollect that the characters of these races go very far deeper--they extend to the bony structure, and to the characters of that most important of all organs to us--the brain; so that, among men belonging to different races, or even within the same race, one man shall have a brain a third, or half, or even seventy per cent.bigger than another; and if you take the whole range of human brains, you will find a variation in some cases of a hundred per cent.
Apart from these variations in the size of the brain, the characters of the skull vary.Thus if I draw the figures of a Mongul and of a Negro head on the blackboard, in the case of the last the breadth would be about seven-tenths, and in the other it would be nine-tenths of the total length.So that you see there is abundant evidence of variation among men in their natural condition.And if you turn to other animals there is just the same thing.The fox, for example, which has a very large geographical distribution all over Europe, and parts of Asia, and on the American Continent, varies greatly.There are mostly large foxes in the North, and smaller ones in the South.In Germany alone, the foresters reckon some eight different sorts.
Of the tiger, no one supposes that there is more than one species; they extend from the hottest parts of Bengal, into the dry, cold, bitter steppes of Siberia, into a latitude of 50 degrees,--so that they may even prey upon the reindeer.These tigers have exceedingly different characteristics, but still they all keep their general features, so that there is no doubt as to their being tigers.The Siberian tiger has a thick fur, a small mane, and a longitudinal stripe down the back, while the tigers of Java and Sumatra differ in many important respects from the tigers of Northern Asia.So lions vary; so birds vary; and so, if you go further back and lower down in creation, you find that fishes vary.In different streams, in the same country even, you will find the trout to be quite different to each other and easily recognisable by those who fish in the particular streams.There is the same differences in leeches; leech collectors can easily point out to you the differences and the peculiarities which you yourself would probably pass by; so with fresh-water mussels; so, in fact, with every animal you can mention.
In plants there is the same kind of variation.Take such a case even as the common bramble.The botanists are all at war about it; some of them wanting to make out that there are many species of it, and others maintaining that they are but many varieties of one species; and they cannot settle to this day which is a species and which is a variety!
So that there can be no doubt whatsoever that any plant and any animal may vary in nature; that varieties may arise in the way I have described,--as spontaneous varieties,--and that those varieties may be perpetuated in the same way that I have shown you spontaneous varieties are perpetuated; I say, therefore, that there can be no doubt as to the origin and perpetuation of varieties in nature.
But the question now is:--Does selection take place in nature? is there anything like the operation of man in exercising selective breeding, taking place in nature? You will observe that, at present, I say nothing about species; I wish to confine myself to the consideration of the production of those natural races which everybody admits to exist.
