MUTATION THEORY OF PROFESSOR BE VRIES. 639 



to explain it. Furthcnuoiv, one can l)y no means foci contidcnt that 

 the utter extinction of that o-ivat sulK-la.ss was due to what has come 

 to be designated as the struggle for existence. ])ecause liefore that 

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

 a])parentlv able to maintain its su])remacy ))oth u])on land and water. 



In the earlier Mesozoic strata remains of fresh-water molluscan 

 faunas are found that contain distincth" modern tj'pes. Among them 

 are species of the true genus Ililo^ which is one of the most widely- 

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

 of species belonging to it arc^ found in other f resh-watcn* deposits of 

 both Mesozoic and Tertiary ag(\ That genus has therefore existed con- 

 tinuously and unchanged during all that stretch of geological time in 

 Avhicii 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 Znio 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 manunals. 

 These apparently had no existence before the l^eginning of Tertiary 

 time, ))ut they then suddenly appeared and assumed faunal dominion 

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

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

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

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

 greater part of its faunal existence, and it seems ni^cessary to assume 

 that the origination of all of them was of a rapid, if not saltator}^ char- 

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

 fresh-water mollusca that hav(? been mentioned, on the other, may be 

 taken as extreme examples of the difference in the chronological ratio 

 of phylogenetic nuitation among organic forms that existed in gcolog 

 ical time. 



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

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

 apparent suddenness that is inconsistent with the theory of natural 

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

 get Dy looking ])ack into geological time may make the periods in 

 which those evolutional changes took place appear shorter than the}" 

 reallv Avere, but a different vicAv would not change the i)rop()rtional 

 elements of the problem. 



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



