NILSSON'S DISCOVERY 91 



various results of systematic research and of morphology, of 

 geography and paleontology, he not only contrived to con- 

 vince the biologists of his time, but also brought the principle 

 of evolution to almost universal acceptance. Especially 

 convincing were his proofs of the numerous adaptations of 

 living organisms to their environments. 



The principle of natural selection was discovered by Dar- 

 win himself. In a general way he showed that many times 

 more individuals are born than can possibly survive. This 

 results in a struggle for life, in which, apart from casualties, 

 those survive which are the best fitted for their special life- 

 conditions. By this struggle the weak are crowded out, and 

 only the best- fitted become the parents of the subsequent 

 generation. This struggle may be seen at work at every 

 moment and on every field. The elimination of the weaker 

 types by stronger ones also has often been observed. But 

 this is nearly always where different species were competing 

 with one another. Concerning the effects of the intraspecific 

 struggle hardly any observed facts are available. Whether 

 it takes an active part in the production of new species, and 

 in what manner it may be able to do so, is a question far 

 beyond our present observation. 



For this reason Darwin relied for a large part on the 

 methods of selection which at his time were in use both in 

 agriculture a'nd in horticulture. He tried to show that the 

 evolution of species at large has followed the same laws that 

 underlie the evolution of races and varieties in culture. In 

 a general way he has succeeded in convincing his contem- 

 poraries of the validity of this analogy. Agricultural and 

 horticultural experience, however, were at that time only im- 

 perfectly developed, and the improvement of races, though 

 successful in a large number of cases, had no really scientific 

 foundation. It did not afford all the evidence required by 

 Darwin for a thoroughly reliable theory. Complying with 



