v NATURAL SELECTION 115 



are the lowest mammals — the echidna and ornithorhynchus of 

 Australia; the lowest birds — the apteryx of New Zealand 

 and the cassowaries of the New -tinea region: while the 

 : fish — the amphioxus or lancelet, is completely isolated. 

 and has apparently survived only by its habit of burrow- 

 ing in the sand The great distinctness ::' the carnivora, 

 ruminants, rodents, whales, bats, and other orders >i 

 mammalia; of the accipitres, pigeons, and parrots, among 

 and of the beetles, bees, flies, and moths, among insects, 

 all indicate an enormous amount :: extinction among the 

 comparatively low forms by which, on any theory of evolution, 

 these higher and more specialised groups must have been 

 preceded. 



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have already seen that, when there is no change in 

 the physical or organic conditions of a country, the efle : : : 

 natural selection is to keep all the species inhabiting it in a 

 state of perfect health and full development, and to preserve 

 the balance that already exists between the different groups 

 of organisms. But. whenever the physical or organic condi- 

 tions change, to however small an extent, some correspond- 

 ing change will be produced in the flora and fauna, since, 

 considering the severe struggle ::: existence ind the ;:znplex 

 relations of the various organisms, it is hardly possible that 

 the change should not be beneficial :: some species and 

 hurtful to others. The most common effect, therefore, will 

 be that some species will increase and others will diminish : 

 and in cases "iiere a species was already small in numbers a 

 further diminution might lead to extinction. This would 

 afford room for the increase :: othei species, and thus a 

 considerable readjustment of the proportions of the several 

 species might take place. When, however, the change was :: 

 a more important character, directly affecting the existence of 

 many species sc as to render it difficult for them to maintain 

 themselves without some considerable change in structure or 

 habits, that change would, in some cases, be brought about by 

 variation and natural selection, and thus new varieties or new 

 species might be formed. We have t: oonsidei then, which 



