DEVELOPMENT HYPOTHESES. 199 



though swimming in the ocean, is nevertheless a mammal 

 breathing by lungs, bringing forth its young alive, and 

 suckling them with true mammalian affection ; and the 

 young - bearing, young - suckling bat, though fluttering 

 through the air like a bird, has no essential feature in 

 common with the birds, save that which belongs to the 

 great vertebrate pattern. Such resemblances are simply 

 adaptive, not essential. Instead of indicating any genetic 

 affinity, they merely point to a law which ordains that 

 agreement of habit and economy, in widely differing groups, 

 shall be accompanied by similarity of form; and this, of 

 physical necessity, so long as the same element has to be 

 traversed, the same kind of food sought after, and the same 

 general functions to be performed. 



Again, if at various stages the lower had given birth to 

 the higher, we should naturally have expected only the 

 lowliest and simplest at first, and an equable and uniform 

 diffusion of the higher races, step by step, in the successive 

 geological epochs. Instead of this, we find protozoans, 

 radiates, articulates, and molluscs, side by side, in the low- 

 est fossiliferous rocks ; and in every stage upwards a 

 variety and complexity of higher and lower, which seem to 

 obey anything but a regular arithmetical or geometrical pro- 

 gression, such as any mere physical law of development 

 must necessarily obey. The palaeozoic brachiopods were 

 higher and more varied than those of existing waters ; the 

 noblest cephalopods shell-clad and shell-less were those 

 of the secondary period ; the highest structural fishes were 

 the sauroids of the upper paleozoic ; and anatomists (Owen) 

 assure us that the thecodont reptiles of the new red sand- 

 stone, had they existed at the present day, would have 

 taken rank at the head of the Lacertian order. So far as 

 paleontology can prove, there is no known line of continu- 

 ous development from one primordial germ no uniform 



N 



