The science of life : fully illustrated in tone and line and including many diagrams

BOOK 93

that is how living things present themselves to-day.

Note the importance of this idea from the point of view of the systematist. In ancient, pre-Evolution days it was believed that living things had been created in definite “kinds” according to divine, but nevertheless presumably intelligible plan. It was our business in classifying living things to detect that plan; one started by assuming that species existed and then tried to find out what they were. Nowadays, on the other hand, we know that living things are a slice through a tree, showing every imaginable degree of cousinship, and not falling tidily and infallibly into “ kinds.” We know that their forms are not constant but changing. And in classifying this assembly, in trying to reduce it to some sort of order and describe it in unambiguous terms, we may choose what conventions we please. Phylum, class, order, family, genus, species, variety—these are words used by general consent to denote the degree of relatedness between forms ; they are not clearly and categorically distinct from each other, but merge together, arbitrary divisions of what is really a continuous series. It is like saying of our tree-section : “ If two twigs are over an inch apart we will put them in different families; if they are over two inches apart in different orders ; if they are over three inches apart in different classes.” And a species, in particular, is no longer a unit created by God, nor is it a natural unit at all like an atom, or a quantum ; it is an arbitrarily defined grouping set up by Man for his own convenience.

From time to time many definitions of species have been put forward, tested and rejected. One only is unassailable. It was proposed by Dr. Tate Regan at a recent meeting of the British Association, and it runs: “A species is a group of animals that has been defined as a species by a competent systematist.” Taken in conjunction with what we have said that definition is perfectly sound. It brings out two essential points—first, that a species is an arbitrary convenience ; second, that an enormous amount of toil on huge numbers of specimens is necessary before a judgment of any value can be reached on a question of classification.

§ 3 The Distribution of Living Things

Everybody knows that different animals come from different countries—the platypus from Australia and Tasmania, the zebra from Africa, the marmoset from South

236

THE SCIENGE OF LIFE

CHAPTER 4

. America, the musk ox from Greenland and

Arctic Ganada, and so on; and the same is, of course, true for plants. But it is not always grasped that different regions differ in respect of whole groups of their animal and plant inhabitants. Contrast the three southern continents: Africa south of the Sahara (that sea of sand which is as much a barrier to life as any sea of water), South America, and Australia. All comprise both temperate and tropical regions; all have their mountains, forests, and open plains. But their animal populations are extremely unlike. If, for the sake of brevity, we restrict ourselves to the mammal population, we find that Africa is characterized by an abundance of antelopes of many kinds, by rhinoceroses, giraffes, elephants, wart-hogs, zebras, lions, leopards, baboons, and buffaloes, and, in the rain-forests, by gorillas and chimpanzees, okapi and many kinds of monkeys. Farther south the coneys or hyraxes and the extraordinary aard-varks are very characteristic. The whole giraffe family, with both giraffe and okapi, is found in no other region, nor is the aard-vark family.

Just as characteristic as the presences are the absences. There are no deer, no beavers, no field-mice or voles, no shrews, no bears, and scarcely any goats or sheep (we are, of course, speaking only of animals found wild).

Contrast this assemblage of mammals with that found in South America. Here live llamas and their relatives, edentates like the sloths, the true ant-eaters and the armadillos, primitive monkeys with prehensile tails, vampire bats, peccaries, tapirs, guinea-pigs, vizcachas and agoutis, opossums. None of these occur in Africa ; and most of them are either wholly restricted to South America, or at the most penetrate a little way into Central or North America. The whole order of the true edentates is confined to this region (Fig. 145).

Finally, Australia (with which for our present purpose we must include the neighbouring islands of Tasmania and New Guinea) is more peculiar still. Before the advent of white men, it contained none of the higher placental mammals whatever, with the exception of bats, whose wings, of course,

give them facilities for spreading denied to

mere land forms, with a few ubiquitous mice and the dingo dog, both probably introduced by the early human immigrants of the country. But by way of compensation Australia possesses a unique menagerie (now, alas, rapidly dwindling, with many species in danger of extermination if protective measures are not introduced) of the two