170 THE THEORY OF EVOLUTION 



wealth of the southern continent was brought to 

 light. The long isolation of South America made it 

 the arena of a vast experiment in evolution, which 

 may be followed in the most gratifying manner and 

 which has solved some of the most puzzling prob- 

 lems of modern distribution. South Africa, Russia, 

 China and the Malay Archipelago have also yielded 

 to the explorer much of which Darwin knew nothing, 

 but which he would have cordially welcomed as 

 strongly supporting his belief. It is the same in 

 other fields of research, new and unexpected dis- 

 coveries are continually being made, but evolution 

 still affords the best explanation of them. A re- 

 markable instance of this is the blood tests, of which 

 an account was given in the third lecture. Here 

 was a totally new field, the existence of which was 

 altogether unsuspected, yet, when the field was 

 opened, it afforded some of the most cogent evidence 

 in favour of the evolutionary conception that has 

 anywhere been found. Not that new difficulties and 

 perplexities have not arisen, but in spite of these, 

 the probability of the theory remains unshaken after 

 more than half a century of unceasingly active 

 investigation carried on all over the world. The 

 doctrine seems stronger now and is upheld by a 

 greater proportion of naturalists than when it made 

 its early conquests of opinion. 



Another searching test of a theory is when it can 

 be made the basis of prediction or deduction. That 

 an astronomer should be able accurately to predict 



