286 Proceedings of the Royal Physical Society. 



kill them, or deprive them of their customary food. That 

 bulk and strength give way to cunning and agility is not 

 peculiar to human activities. The tiger who sees the carcase 

 on which he has just feasted devoured by birds has no power 

 of retaliating on the thieves, because his superior strength 

 cannot be brought into action against his aerial foes. 



The result of this brief survey of the organic world, past 

 and present, is to show that while the vast majority of living 

 things have not risen high up in the scale of evolution — 

 some remaining for ages on a comparatively low plane, or 

 even degenerating, and others becoming extinct — there are a 

 few which have made extraordinary advances both as regards 

 differentiation of structure and specialisation in function. 

 That the chronological order in which animals flourished 

 in by-gone ages was from the less to the more highly 

 developed will be apparent by a glance at a table of the 

 stratified rocks, with their typical fossil remains. Starting 

 with the Palaeozoic period, and passing in succession through 

 the more recent strata of the earth's crust, we find the 

 sequence in the appearance of the Vertebrata to be — fishes, 

 reptiles, birds, mammals, and man. That is to say, in the 

 Cambrian period only fossils of invertebrate animals are 

 found; in the Devonian we have, in addition to these, 

 remains of fishes ; while in the lower Mezozoic we have, 

 together with all the previous forms, those of reptiles. In 

 the Cretaceous period birds and mammals appear for the 

 first time among the inhabitants of the globe, while man, the 

 most highly developed of all, comes on the scene only 

 towards the end of the Tertiaries. 



This chronological sequence in the development of animal 

 life has a remarkable parallel in the phenomena of embry- 

 ology ; and this to my mind is a striking proof of the 

 truth of the evolution theory of organic life. In embryology 

 the ultimate visible and starting-point in the development of 

 the animal is the ovum, which is virtually a cell or living 

 corpuscle. So complete is the parallelism observed in the 

 series of changes which take place during foetal life, and the 

 gradual development of animals into the higher stages of 

 existence, that Hackel formulated the theory, that the develop- 



