MATTHEW, CLIMATE AND EVOLUTION 279 



but the evidence that they actually did so survive is open to serious 

 question. So far as they go, the facts accord v^^ith the dispersal of the 

 dinosaurs from the northern land mass. And so far as I have been able 

 to review the data, the migrations of the order could be made to conform 

 with the present distribution of continental and abyssal areas (Mada- 

 gascar excepted^"^) about as well as with the different distribution upon 

 which they are plotted by Dr. Lull. 



It is significant in this connection to note that young individuals are 

 very rarely found in the dinosaur formations. Thousands of individuals 

 are found together in some of the great quarries, pertaining to a great 

 number and variety of genera and with a wide range in size, but it is 

 very rare to find young individuals among them. This fact is well known 

 to collectors, but has not, as far as I know, been commented upon in 

 print. It is true that young individuals are less clearly distinguished 

 from adult among reptiles than among mammals, the chief difference 

 being the imperfect ossification of the bone structure, and that such im- 

 perfectly ossified bones are likely to be poorly preserved and might often 

 be rejected by collectors on this account. But making all reasonable 

 allowance for these considerations, there remains a very notable contrast 

 with fossil mammal quarries and fossiliferous formations, in which young 

 individuals are always to be found among any considerable number of 

 adult specimens and often are more numerous than mature individuals. 



This may be interpreted in conformity with the above theories as to 

 the habitat of dinosaurs, by supposing that the young dinosaurs were 

 more dry land or upland animals, retaining the ancestral habitat, and 

 coming down into the swamps only when they reached maturity and 

 their larger size made an amphibious or aquatic habitat more suitable. 

 The young animals would rarely or never visit the swamps and deltas, 

 whose formations have alone been preserved, and their fossil remains 

 would be correspondingly scarce. 



^oung crocodiles, so far as I can gather from various descriptions, are 

 somewhat more terrestrial in habit than the full-grown animal, but the 

 difference is evidently not considerable. Analogous cases among fish, 

 marine types breeding in fresh water and vice versa, are well known. 

 The migration of birds has also some analogy, if, as may often have been 

 the case, the swamp dinosaurs resorted to dry land for breeding and egg- 

 laying purposes. In either case the breeding or egg-laying place would 

 be presumptively the ancestral habitat of the race. 



"3 The Cretaceous sauropoda of Madagascar may have reached that island in the same 

 manner as the hippopotamus did at a later period, namely by swimming. 



