INDIRECT EVIDENCE. 45 



only evidence is the fact that domestic races are 

 fertile when crossed. Considering all that is known 

 as to the fertility of hybrids, this is certainly too 

 slim a basis to authorize the assumption of the 

 whole question. 



Indirect Evidence. 



Not having time enough at their disposal and not 

 being able to define the term species, naturalists are 

 still unable to decide by direct evidence whether or 

 not species are immutable. If the evidence that 

 species have been derived from each other ceased 

 here, few would be inclined to accept evolution. 

 But there is a large amount of indirect evidence 

 bearing upon the question. If species have arisen 

 by the process above described, certain relations 

 would necessarily follow. It is the exact accord of 

 all the evidence collected with these necessary rela- 

 tions, which has caused such a wide acceptance of 

 organic evolution. Such evidence is indirect, but it 

 is none the less forcible. In the following four 

 chapters this evidence will be chiefly discussed, but 

 a word of preliminary introduction is here desirable. 



When we come to examine the relations of true 

 species, we find many points which suggest genetic 

 relation. As already mentioned, no naturalists have 

 been able to distinguish in nature between species 

 and varieties. All grades can be found between 

 two similar varieties and two very distinct species. 

 Indeed among the sponges careful students have 

 been unable to find any thing which they can call 

 species ; for the whole group is connected together 



