252 THE PHILOSOPHY OF BIOLOGY 



trace these changes of morphology, aided by the study 

 of embryology and by the comparison of the structure 

 of the parts of fossil animals. Regarding the process 

 of transf ormism as proved by experiments and observa- 

 tions in breeding and heredity, the naturalist endeavours 

 to trace the lines along which evolution has proceeded 

 from the results of morphological investigations. 



Such results cannot have more than a very limited 

 value, and it is often the case that several interpreta- 

 tions of morphological results are equally probable. 

 We may conclude that the existing Teleost and Elas- 

 mobranch fishes are descended from a common stock 

 which no longer exists ; we may similarly conclude 

 that the Birds and Reptiles are closely allied, more so 

 than either group is to the Mammals ; and we may 

 conclude that the Primates the group of Mammals to 

 which Man belongs is descended from some group 

 allied to the existing Ungulates or Insectivores, while 

 the Mammals themselves may have come down from 

 some group of vertebrates related to both the Amphibia 

 and the Reptiles. But as to the nature of the animals 

 which combined the characters of the Birds and Reptiles, 

 or of the Reptiles and Amphibia, we know nothing. 

 Palaeontology, if its results were more numerous than 

 they are, would afford us the material for the discovery 

 of these " missing links," and there can be no doubt 

 that as the world becomes better known our knowledge 

 of palaeontological stages in the history of existing 

 groups will become more complete, so that we may, in 

 time, possess an actual historical record of the phylo- 

 geny of the main groups of animals. But it is remark- 

 able that while the results of comparative anatomy 

 and embryology, aided by those of palaeontology, 

 enable us to trace back short series of stages in the 

 evolutionary process, they still show us gaps at all the 



