3 6 INTRODUCTION 



must have given rise to the host of species that once existed, or that 

 now exist, for the plants now growing on the surface of the earth are 

 only the twigs and terminal branches of an enormous genealogical tree. In 

 the development and evolutionary history of plants, competition and 

 selection have always played a prominent part in determining whether 

 variations in any given direction were maintained and developed, or 

 suppressed. Whatever part these and other factors may play, the final 

 result is the same ; for only those variations which are advantageous to the 

 organism can be permanently retained. 



So long as every living organism is found to arise from pre-existent 

 life, the riddle still remains unsolved as to how life first came into existence 

 upon this earth. Nor is it possible for us to say whether the path which 

 leads to the source of life is lost in the obscurity of infinite space, or 

 whether living material was first formed from dead substance on our 

 planet. And however much we may be disposed to believe the latter to 

 be the case, it nevertheless remains uncertain whether the conditions now 

 existing upon our globe are such as to permit a re-creation of life, or 

 whether the necessary conditions were presented only once, and by a special 

 sequence of events, such as we can never hope to reproduce. The particular 

 combinations of causes, to which the creation of life was possibly due, 

 may have existed only as the earth cooled from its original incandescent 

 condition, and perhaps thereby caused certain essential preliminary stages 

 in the production of living substance to arise. 



As growth and reproduction go on, the living substance must con- 

 tinually assimilate dead material. The same inherent structure and con- 

 stellation are, however, retained by the protoplasm in spite of the perpetual 

 change. Nevertheless it is possible that a descendant may not contain 

 a single one of the same atoms that once formed part of a direct ancestor. 



Death and destruction, as well as growth and reproduction, are continu- 

 ally taking place. As the resultant of these opposing factors about the same 

 average quantity of living substance is maintained upon the earth so long 

 as the conditions remain constant, but this state of approximate equilibrium 

 will necessarily be disturbed and altered when any alteration takes place 

 in the external conditions 1 . 



So long as the earth was in a molten condition, no life such as is now 

 known to us could have existed upon it; while, on the other hand, the 

 gradual diminution of life, as we proceed towards the Arctic regions, shows 

 us that the present total aggregate of living substance would undergo 

 a corresponding decrease, if similar climatic conditions held their sway over 

 the entire earth. 



1 Preyer, Nat. Wochenschr., 1891, Bd. vi, p. 92. A good criticism by Errera, Rev. Phil., 

 1891, p. 322. 



