NATURAL SELECTION 115 



are the lowest mammals — the echidna and ornithorhynchus of 

 Australia; the lowest birds^ — the apteiyx of New Zealand 

 and the cassowaries of the New Guinea region ; while the 

 loAvest 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 of the carnivora, 

 ruminants, rodents, Avhales, bats, and other orders of 

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

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

 all indicate an enormous amount of extinction amonsr the 

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

 these higher and more specialised groups must have been 

 preceded. 



Circumstances favourable to the Origin of New Species hy 

 Natural Selection. 



We have already seen that, when there is no change in 

 the physical or organic conditions of a country, the effect of 

 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 difi'erent 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 for existence and the complex 

 relations of the various organisms, it is hardly possible that 

 the change should not be beneficial to some species and 

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

 be that some species Avill increase and others -will diminish ; 

 and in cases where a species was already small in numbers a 

 further diminution might lead to extinction. This would 

 afford room for the increase of other species, and thus a 

 considerable readjustment of the proportions of the several 

 species might take place. "When, hoAvever, the change was of 

 a more important character, dii-ectly affecting the existence of 

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

 themselves without some considerable change iu 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 to consider, then, which 



