4 INTRODLC7ORY 



another, and even one of the greater groups and others more or 

 less distinctly separated. This is exactly the condition that we 

 find to exist, and as we descend from the higher and more 

 specialized forms to those that are lower and simpler, we find these 

 connecting links more and more pronounced and connecting larger 

 and larger groups of organisms. Classification instead of being 

 simply a means of separating forms, has become a method of 

 studying affinities, and tracing the phylogenies or race histories 

 of groups of organisms throughout their complicated alliances. 



With the understanding then that no hard and fast lines can be 

 drawn between any of the greater groups of living things, we will 

 endeavor to show the three lines of evolution which have been at 

 work and have given us three types of organisms each with its 

 peculiar tendencies leading it toward a different goal. 



Whether an organism be high or low, there are certain common 

 principles involved in its existence. In the first place it finds itself 

 face to face with a tremendous struggle for existence, and secondly 

 it puts forth a powerful effort not only to maintain its life, but also 

 to perpetuate its kind. A single garden weed produces a thousand 

 or even ten thousand seeds, and yet most weeds are not promi- 

 nently increasing in the total number of individuals from year to 

 year ; a plant of the rare hart's tongue fern produces perhaps 

 50,000,000 spores in a season and yet is barely holding its own in 

 the struggle for existence, and in our own country is found only in 

 a few favored ledges of rock mostly of a certain geologic period ; 

 the spores of the giant puff-ball number untold millions and each 

 microscopic spore under favoring conditions would be able to pro- 

 duce a new plant, but the giant puff-ball is far from being com- 

 mon. When we consider the fact that each plant in its lifetime 

 has before it the necessity to produce only one seed or spore that 

 shall come to maturity, in order that the plant may hold its own, 

 and when we consider, moreover, that more plants are growing 

 scarcer than are increasing in number, we can comprehend some- 

 thing of the terrible struggle for existence that is everywhere and 

 always in progress and faces every living thing from monad to 

 man, from the instant it appears on the face of the earth. 



To perpetuate its kind, then, is the first great instinct of a liv- 

 ing thing and methods of reproduction are the functions devel- 

 oped simultaneously with assimilation and growth. Among all 



