iSgi.] 



THE BRITISH NATURALIST. 



25 



whose gigantic size and ferocity made them the terror of the sea. We 

 are not, however, concerned with these marine forms, but with their 

 terrestrial cousins, some of which began to shew distinctly bird-like 

 structures in various parts of the skeleton. Some of these reptiles 

 attained an enormous size, and presented most curious and almost in- 

 credible modifications of structure. 



The Iguanodon, with a body about 25 feet long, habitually assumed 

 an erect attitude. It is difficult to picture to ourselves the appearance 

 of a creature much heavier than a hippopotamus, and longer than a 

 crocodile, sitting and moving in kangaroo fashion. That such creatures 

 have existed there is no doubt, for we find their remains, and from 

 them can build up the skeletons as certainly as those of existing 

 reptiles. As far as can be judged from the skeletons, most of these 

 huge reptiles had air-sacs extending into the bones, thus lightening, 

 without weakening, the framework, just as is the case with most birds. 

 It would be extravagant to suppose that a reptile ever directly pro- 

 duced a bird ; but it is obvious that all existing reptiles and birds are 

 descended from a common reptilian ancestor. So far, the order of the 

 fossils from the carboniferous, through the Permian, Trias, and Lias, 

 to the Oolite period, has shewn us an unbroken chain— from the lower 

 to the higher — from the newt to the highly specialized reptile, though, 

 till this Oolite period, there are no fossils of true flying reptiles or 

 birds. You will note that the facts so far accord with the doctrine of 

 descent ; if, on the contrary, the higher preceded the lower — that is to 

 say, if it could be shewn that birds occurred before reptiles, the 

 evolutionary theory would fall to the ground ; but the more knowledge 

 we gain the more certain does it become that the lower has always 

 preceded the higher, and that the doctrine of descent is true. Of 

 course, you must not think that the lower forms have any desire to 

 become other than what they are, their modifications arise from vari- 

 ation, are perpetuated under the laws of heredity and natural selection, 

 of which they have no consciousness ; but in the struggle for existence, 

 those only which can adapt theni2:elves to their surroundings will 

 survive and multiply. 



It is not asserted that the environment makes the organism, though 

 it is scarcely possible to lay too much stress on its importance in 

 modifying it. The most trifling change in the surroundings of either 

 plant or animal must produce some effect on that plant or animal, 



