NORTH AMERICA AND THEIR VERTEBRATE FAUNA. II3 



development by the attainment of a particular form of isolation which 

 relieved it from the necessity of struggle. In this case we suppose the animal 

 to have developed certain characters which enabled it to triumph easily over 

 any unfavorable influences (contacts). 



As I have reviewed the fauna time after time I have failed to discover 

 any animal which could in any way compare with Dlmetrodon in agility and 

 strength, or in its adaptation to raptorial habits. The genus seems to be a 

 splendid example of the development of a single group to an ascendency over 

 all its contemporaries. The only active competition which could have oc- 

 curred would have been between animals of the same genus, since the other 

 members of the fauna were either harmless in habit or far too small to have 

 been effective enemies. There was an abundance of food, easily obtained, 

 no enemies of equal power, and an apparently suitable inorganic environ- 

 ment, svirface, climate, etc. In other words, the animal was, so far as we can 

 evaluate the factors, very perfectly adapted to its environment. (Of course, 

 there may have been antagonistic elements of which we have no knowledge.) 

 It had reached what Gratcap has called the "zoic maximum." 



I may here quote Beecher's words " on the effect produced upon a group 

 by a very complete adaptation to its surroundings. 



' ' The prolonged development or existence of a stock under favorable conditions 

 for multiplication may he considered as one of the primary influences favoring the 

 production of spines. This implies abundance of nutrition and comparatively few 

 enemies outside of other individuals of the same or closely related species. Under a 

 proper amount of increased nutrition, the vitality and reproductiveness of a stock 

 are raised, and, other things being favorable, it is found that the stock will give 

 expression to what has already been described as free variation. Hypertrophy is 

 also very apt to be one result of abundant nutrition, so that structures of little or 

 no use may be developed, and some of them comprise certain features which are 

 often called ornamental. 



"bi the excessive multiplication of individuals, it is evident that there must be 

 a great number of natural variations, and that some of these will affect the pairing 

 of the sexes in such a manner as to accentuate and delimit certain variations. 

 Eventually, there also comes a struggle for existence in which favorable modifica- 

 tions have a decided advantage. In this way it is believed that the great amount of 

 differentiation found in some isolated stocks has been brought about. Primarily, 

 then, a favorable condition for nutrition is assumed, which is followed by excessive 

 numerical multiplication, while the natural variations are augmented and governed 

 by the action of reproductive divergence for which such conditions are favorable. 

 Secondarily, these variations are subjected to the influences of cannibalistic selection, 

 defense, offense, sexual selection, and mimicry. 



' ' In illustration of the amount of differentiation attained by a single stock under 

 favorable conditions, the Amphipod Crustaceans, Gammarus and Allorchestes, found 

 in Lakes Baikal and Titicaca, respectively, may again be noticed. 



"In respect to the number of species, Gammarus is very sparsely distributed 

 over the world, though in Lake Baikal alone 117 species have been described by 



• Beecher, Amer. Jour. Sci., vol. vii, 1898, pp. 266-268. 



