MUTATION THEORY OF PROFESSOR BE VRIES. 639 



to explain it. Furthermore, one can by no means feel confident that 

 the utter extinction of that great subclass was due to what has come 

 to be designated as the .struggle for existence 1 , because before that 

 occurrence it had long ruled the animal life of the earth and was 

 apparently able to maintain its supremacy both upon land and water. 



In the earlier Mesozoic strata remains of fresh-water molluscan 

 faunas are found that contain distinctly modern types. Among them 

 are species of the true genus Uhio, which is one of the most widely 

 dispersed and characteristic fresh-water genera now living, and remains 

 of species belonging to it are found in other fresh-water deposits of 

 both Mesozoic and Tertiary age. That genus has therefore existed con- 

 tinuously and unchanged during all that stretch of geological time in 

 which all the mammals, all the birds, all the teleost fishes, and all the 

 exogenous plants of the earth were introduced and in which the dino- 

 saurs culminated and became extinct. Those earliest known speci- 

 mens of the genus Unio are fully characteristic, but we know nothing 

 whatever of their origination or of any earlier related forms. It seems 

 impossible to assume that this genus was not suddenly produced, and 

 it seems equally evident that upon its introduction it passed at once 

 into its immutable state, which has continued until now, at least in a 

 main line. 



The case seems to have been very different with other forms of ani- 

 mal life that are now extinct, especially with the placental mammals. 

 These apparently had no existence before the beginning of Tertiary 

 time, but the} T then suddenly appeared and assumed faunal dominion 

 of the earth in forms nearly or quite as highly organized and diverse as 

 are those which now exist; but every one of those earlier mammalian 

 forms, with many others of later origin, are now extinct. The muta- 

 tive period of each of those forms was probably coeval with at least the 

 greater part of its faunal existence, and it seems necessary to assume 

 that the origination of all of them was of a rapid, if not saltatory char- 

 acter. The case of these mammals, on the one hand, and that of the 

 fresh-water mollusca that hare been mentioned, on the other, may be 

 taken as extreme examples of the difference in the chronological ratio 

 of phylogenetic mutation among organic forms that existed in geolog 

 ical time. 



Not only the placental mammals, but the birds of modern types, the 

 teleost lishes, and the exogenous plants, were also introduced with an 

 apparent suddenness that is inconsistent with the theory of natural 

 selection. It is true that the foreshortened view which we necessarily 

 get by looking back into geological time may make the periods in 

 which those evolutional changes took place appear shorter than they 

 really were, but a different view would not change the proportional 

 elements of the problem. 



One of the strongest arguments that have been used in support of 



