BOTANY 



attaining its mature condition, its ontogeny. The supposition, that 

 the successive steps in the ontogenetic development of an organism 

 correspond to those of its phylogenetic development, and that the 

 ontogeny of an organism is accordingly a more or less complete 

 repetition of its phylogeny, was asserted by Fritz Muller (-), Avho 

 based his conclusions on the results of comparative research. 



The idea of the gradual evolution of higher organisms from lower 

 was familiar to the Greek philosophers, but a scientific basis was first 

 given to this hypothesis in the last century. Through the work of 

 Charles Darwin (■'') in particular, who accumulated evidence for a 

 reconsideration of the whole problem of organic evolution, the belief 

 in the immutability of species has been overturned. 



Darwin sought to explain the origin of species by natural causes, 

 laying chief weight on a process of selection. In drawing his con- 

 clusions, he proceeded from the variability of living organisms, as 

 shoMHi by the fact that the offspring neither exactly resemble their 

 parents nor each other. Further, he called attention to the constant 

 over-pioduction of offspring, the majority of which must inevitably 

 be destroyed. If this were not so, and all the young produced by 

 even a single pair attained their full development, they would, in a 

 few generations, completely cover the whole surface of the earth. 

 On account of insufficient space for all, the different claimants are 

 engaged in an uninterrupted struggle, in which the victory is gained 

 by those that, for any reason, have an advantage. Through this 

 " struggle for existence " a selective process goes on among the 

 characters appearing in individual variations, and those which under 

 the conditions of life are in any way advantageous tend to be pre- 

 served. In this manner Darwin arrived at the supposition of a 

 process of natural selection, which is the essential of his theory. 

 Newly developed peculiarities must be inherited in order to become 

 permanent characteristics of a later generation. Darwin sought in 

 the experience of breeders evidence that such characters are inherited. 

 The breeder selects individuals presenting any desired characters for 

 the purpose of breeding, and has thus formed the races of domesticated 

 animals and cultivated plants. These have often departed so widely 

 from their wild ancestral forms that the latter are not certainly known. 

 Natural selection will work in the same way as this artificial selection 

 and, by repeated choice of individual peculiarities, which can be 

 inherited and thus combined with the pre-existing characters, can give 

 rise to new forms ; these ultimately may depart widely from the 

 ancestral form. By the continued operation of natural selection, 

 organisms must result, which are, in the highest degree, fitted and 

 adapted to their environment. In this way the theory of selection 

 seeks to explain as due to natural causes that adaptability to the 

 environment which is such a striking characteristic of organic life. 

 That the transitional forms in this process of phylogenetic development 



