130 THE BEGINNINGS OF LIFE. 



to go through essentially similar changes. Again, 

 since nobody can know how rapidly the various lower 

 ^specific' forms were evolved upon the surface of the 

 earth in past times — and even in all subsequent periods 

 preceding our own — the experimental evidence and 

 the knowledge derived from actual observation which 

 we now possess concerning the present comparatively 

 rapid appearance of specific forms, must be regarded 

 as so many contributions to our positive knowledge 

 upon the subject, which far outweigh in value all the 

 a priori deductions of even the profoundest reasoners. 



Observation and experiment combined reveal the 

 fact that new-born specks of living matter, after 

 emerging into the region of the visible, continue to 

 increase in size and soon exhibit signs of a primary 

 differentiation. But the rapidity with which any 

 degree of complexity of organization manifests itself 

 will be found to vary very much with the mode of 

 nutrition of the living aggregate, and with its manner 

 of growth. The lower Fungi, Algse, and Lichens are 

 produced in great part by a mere *" irrelative repetition ' 

 of similar parts — though their growth may be either 

 continuous or discontinuous \ But the nutrition of 

 all these lower forms of life is effected by a synthesis 

 of new living matter from not-living elements, and 

 they further agree amongst themselves in manifesting 

 a comparatively slow rate of variation. On the other 

 hand, in Amoebic, Flagellated Monads, Ciliated Infu- 



^ See note, p. 92. 



