1895.] NATURAL SCIENCES OF PHILADELPHIA. 257 



to these influences. Nature lias been frequently credited with this 

 destructive work, some world-wide convulsion being called in to do 

 duty as an efficient agent. But this cataclysmic theory has been 

 largely over-employed, and could hardly have confined its ravages to 

 the larger reptiles, of sea and 'land alike, while leaving the smaller 

 reptiles and the contemporary mammals unharmed. 



In seeking to discover some adequate cause for so great a natural 

 event, one destructive agency, not as yet" mentioned, offers itself as a 

 not improbable explanation. It is one neither of inorganic action, of 

 food competition nor of direct assault. On the contrary it is a kind 

 of indirect assault — an assault not on the animals themselves, but on 

 their eggs and young. This destructive influence is one that is very 

 prevalent in the animal world. It is efficient in keeping down the 

 numbers of prolific forms at present, and may have had much to do 

 with the extinction of species in the past. It is a danger to which 

 the mammalia are exposed only in the case of their young, and in 

 this case only to a minor degree, from their vigilant care of their 

 young; but to which the fish, reptiles and birds are exposed in the 

 case of their eggs as well. The evolution of instinct has taught 

 birds to care for their eggs and young, and thus in great measure to 

 escape this peril. In reptiles this instinct of carefulness is very 

 little developed, and in fishes scarcely at all. Fish species, indeed, 

 eseape annihilation mainly through fecundity. Though myriads of 

 their eggs and young are devoured, enough escape to ensure the con- 

 tinuance of the species. The reptiles are intermediate between the 

 birds and the fishes in these particulars, less prolific than the latter, 

 less careful than the former. Existing reptiles take little or no care 

 of their young and rarely any special care of their eggs. The 

 turtles conceal theirs very skilfully in the sand and leave them t<> 

 chance and secrecy for safety; the young, when hatched, being very 

 agile in their escape to the water. But eggs and young alike have 

 their enemies. The former are often discovered and devoured; the 

 latter have numerous foes in and out of the water. Only a mere 

 fraction of the brood escapes to keep alive the species. The crocodile 

 lays its eggs in the warm sand, or in a heap of mud or decaying 

 vegetation, and pays no further attention to them. The alligator is 

 more careful, keeping some measure of watch and ward over its eggs. 

 The existing laud reptiles — the snakes, lizards and land turtles — as 

 a rule, pay but little attention to the fate of their eggs and young, 



