PRINCIPLES AND METHODS OF PALAEONTOLOGY. 375 



found but a very short way beyond tide marks. The h)wer limit of the 

 plant Nidlipora, on the other hand, seems to mark in all seas the line of 

 demar(%ation between moderate depths (under one hundred fathoms) and 

 great deptlis. 



We must remember, however, in attempting to a])ply these geuerali 

 zations, that as yet distribution in depth has hardly been fairly worked 

 out, even in temperate latitudes, and that before we can safely enunciate 

 laws of general application, a vast number of observations must be made 

 in l)otli tropical and aix'tic climates. 



The fact of the ai)parently capricious limits which have been assigned 

 to man^' animals has been alluded to above. That all animals are 

 adapted to the conditions in which they live is a truism, for if they were 

 not so ada])ted they would not live, but die; but the strange fact is that 

 we do not always tind anjmals in those conditions for whicli they are 

 adapted. At the present day millions of horses run wild over the Pampas 

 of South America, and these great plains are overspread svith a peculiar 

 kind of thistle ; there can be no doubt, therefore, that the climatal and 

 other conditions of this part of the American continent are eminently fa- 

 vorable to both horses and thistles. Nevertheless, at the period of the 

 discovery of the Americas, neither the horse nor the thistles existed in 

 these regions. 



In like manner, eighty years ago, neither horse, nor ox, nor sheep 

 grazed the wide pastures of Australia; now they flourish, and run wild 

 there. The same is true of i^ew Zealand. The little fresh-water muscle, 

 the Dreissena, now so common in our canals, ha%iug swarmed over 

 the whole country, is a recent importation from Eastern Europe. Con- 

 ditions most favorable for its existence have existed for ages, and yet it 

 only now reaches th(Mn artificially. However trite may be the assertion, 

 therefore, that animals are fitted for their conditions, the converse propo- 

 sition, that conditions imply the existence of creatures fitted to flourish 

 in them, is manifestly untrue. 



Again, the existing distribution of animal life furnishes good grounds 

 for exercising the greatest caution in reasoning from the population of 

 one area, however vast, to that of another. A naturalist might be per- 

 fectly acquainted with the indigenous animal inhabitants of all South 

 America and Australia, and yet not know that there were such things in 

 the world as the elephant, the rhinoceros, the hii)popotamus, the giraffe, 

 the lion, the tiger, the horse, the ox, the sheep, or the goat, lie might 

 be fully ac(]uainted with the population of all the enormous area which 

 contains Australia and the Pacific Islands, and yet not only be ignorant 

 of the animals just mentioned, but might never even have heard of bears, 

 cats, monkeys, ruminants, sloths, or ant eaters. Finally, the exclusively 

 African naturalist might fairly conclude from his own experience that 

 great quadrupeds abound everywhere, and that there are no such things 

 as kangaroos or opossums. 



The commonest facts in distribution, therefore, teach us that it is 

 never safe to apply conclusions based upon the investigation of a limited 

 area, however large, to the animal inhabitants of the rest of the world. 



There is yet another caution necessary in reasoning from the facts of 

 distribution. It should be well borne in mind that the connection between 

 a given form and the conditions in which that form flourishes is, in tlie 

 great majority of cases, unknown to us. The laws of distribution are 

 for the most part purely empirical; they are merely the expression of 

 observed facts, of the reason of which we know nothing. If we observe 

 species A always in a warm climate and species B always in a cold one, 

 we ma3' conclude if we find specimens of A and B that the climates iji 



