THE BEGINNINGS OF LIFE. 639 



must, in each period, have given rise to innumerable 

 multitudes of what have been called 'trees of life,' branch- 

 ing out into animal and vegetal forms of almost incon- 

 ceivable variety. Myriads of these 'trees,' including 

 all their branches and innumerable ramifications, may 

 have wholly died out during the many vicissitudes of 

 the earth's surface and the long lapse of ever-fruitful 

 ages • though the descendants or ultimate ramifications 

 of some of such trees — dating back to quite different 

 and perhaps far-distant epochs — may still survive 

 upon the earth's surface. How far, however, the 

 roots of any of those trees from which the existing 

 higher forms of life are derived, may have extended 

 back into the depths of geologic time, we are utterly 

 unable to estimate. 



Throughout all this life -evolving period of the 

 history of our globe, the progress of ' organization ' 

 seems to have been essentially similar. And that this 

 should be so, seems readily explicable by the con- 

 sideration that living things, both as regards their 

 origin and their subsequent differentiation or deve- 

 lopment, are the immediate products of ever-acting 

 natural laws or material properties which are probably 

 the same now as they have ever been. 



The lower the forms of life — th^t is the nearer they 

 are to their source — the greater seems to have been 

 the similarity amongst those which have been produced 

 in different ages. On the other hand, the longer any 

 particular tree of life has lived (of which there have been 



