306 CONCLUSION 



that it has never in a single instance reappeared in the same pool. 

 At Dolgelly, where also in some years it is common, I met with 

 the same result, with a single exception, when I gathered it in one 

 pool for two successive years. I have noticed the same fact with 

 regard to Zyngeina curvatum, and I believe it holds good in regard 

 to most if not all the other Conjugatae." 



In the face, then, of this absence of continuity of habitat in the 

 present, and of the frequent extreme variability of the lower forms 

 of life — some indications of which have been given in the fore- 

 going quotations from F. Cohn, Carpenter and others, together 

 with what has been said as to the variability of Algae generally 

 (pp. 289-291) — how is it possible to accept the notion of direct 

 lineal descent without change, as the explanation of " persistence 

 of types " ? 



As I long ago pointed out' persistence of low types of life is 

 much more explicable on " the assumption of successive evolutions 

 of more or less similar forms from similar starting points, under 

 the influence of like conditions, than on the assumption that such 

 changeable forms should have produced their like through such 

 vast and unrealisable epochs of time." Persistence of types 

 among lower forms of life is, in fact, to be expected in accordance 

 with my views, seeing that the living things that have been con- 

 stantly arising by Archebiosis and Heterogenesis have been the 

 immediate products of ever acting material properties or natural 

 laws — the same in all times, however much or little the environing 

 conditions may have varied from age to age. 



Thus, the continued existence of low types throughout the 

 geologic strata from the Silurian system upwards ; and, among 

 higher types, the constant admixture of previously known forms 

 with others altogether new, will be found quite consistent with the 

 notion of a continual surging up through all geologic time of 

 freshly evolved, lower forms of life — representatives of which, as 

 they become more and more highly organised, mix, in successive 

 epochs, with those of their predecessors which still remain. In 

 this way there may have been produced in successive geological 

 ages, in different parts of the earth's surface, multitudes of what 

 have been called " trees of life " branching out into animal 

 and vegetal forms of almost inconceivable variety. Many of 

 these trees, including all or most of their branches, may have died 

 out during the many vicissitudes of the earth's surface and the long 

 ■ " The Beginnings of Life," vol. ii., p. 616. 



