550 



HISTORICAL GEOLOGY 



dominance of mammals on the uplands and, although the dinosaurs 

 held on in the swamp regions and had adapted themselves more or 

 less to the new vegetation, they had probably become extinct in the 

 uplands long before the close of the period. When the elevation that 

 divides the Mesozoic from the Tertiary occurred, it caused the disap- 

 pearance of this swamp fauna, but in the following period (Tertiary) 

 we find a swamp fauna being developed again, not from upland 

 dinosaurs but from the mammals which had taken their place (p. 590). 



Size as a Factor in Extinction. — "It is a well-known mechanical principle that the 

 strength of a beam varies in proportion to its cross section ; its weight in proportion 

 to its mass. Hence, a beam twice as large lineally as another of the same shape will 

 be four times as strong and eight times as heavy. Its strength, in proportion to its 

 weight, varies inversely as its lineal dimensions. Or, to have a beam support a load 

 proportioned to, its length, its diameter must be increased by V2 for every doubling of 

 lineal dimensions. 



"Apply this principle to the skeletons and muscles of animals, and it will appear 

 that the bones must become more massive and the muscles (whose strength of pull 

 varies with their cross section) heavier with increase of size in the above proportion. 

 But the proportionately heavier muscles must mean a proportionately greater amount 

 of food required to supply power. If one animal is twice as large lineally as another, 

 the length of its limbs will be twice as great, but its weight will be eight times as great. 

 In order to support that weight, the bones and muscles must be eight times as strong. 

 Since their strength depends upon their cross section, their diameter must be V8 times 

 as large. To move the greater bulk of the larger animal will require, on account of 

 its more massive build, somewhat more than eight times as much expenditure of 

 energy. This energy is supplied from the food which it finds in its path. Now the 

 larger animal, supposing its movements to be in proportion to its size, will traverse 

 a path which will be twice as long and its reach will be twice as wide. In other words, 

 the larger animal, with the expenditure of eight times as much energy, will cover a 

 food area four times as large as that covered by the smaller one. If conditions be 

 equal, it will find and secure four times as much food in the same length of time, but 

 as we have seen, it will consume more than eight times as much energy in doing so. 

 From this it will follow that the larger animal must use more than twice as much time 

 in securing the necessary food to maintain its activities as the animal half as large in 

 lineal dimensions. 



"Quite obviously, this will fix a definite limit of size which will be reached when the 

 animal expends practically all of its time in securing and eating food. After that 

 point is reached, further increase in size can be obtained only when : (1) food becomes 

 more abundant; (2) the race becomes adapted to a more abundant but hitherto un- 

 suitable kind of food ; (3) new adaptations are evolved for more rapid securing and 

 digesting of food ; (4) the animal is relieved of the support of part, or of most of its 

 weight, by adopting an aquatic life. It is then subject to a new series of conditions 

 involving a limit of size, indeed, but a much higher one than for terrestrial animals. 

 It is a matter of common observation that while very large animals spend nearly all 

 their time in eating, small animals spend a small proportion of theirs, and most of it in 

 other activities. 



