THE MASTER-KEY TO ZOOLOGY. 87 



among the reptiles, the crocodiles, alligators, and turtles 

 are aquatic, while the flying dragon .skims through the 

 air after the manner of the petaurists and the flying 

 squirrels ; while there are even fishes, and fishes not a 

 few, which at times leave their natural element and 

 trespass for awhile upon the land. All these, and many 

 more, are described in " Trespassers," which not only 

 gives an account of the animals themselves, but explains 

 the wonderful modifications of structure which enable 

 them to " trespass." In this book, in fact, my father 

 first dilated upon that great and most important fact 

 the one and the only master-key to all zoology, that 

 Structure is subservient to and dependent upon Habit ; 

 in other words, that every detail of the bodily form of 

 every animal is more or less modified in accordance with 

 the life which that animal is intended to lead. And 

 upon this rule one to which there is no exception he 

 was never afterwards tired of insisting, alike in book, 

 in magazine article, in sketch-lecture, and even in 

 sermon. 



Almost contemporary with " Trespassers " was " Out 

 of Doors " ; but this was merely a collection of magazine 

 articles which had from time to time appeared in various 

 periodicals, and which were now deemed worthy of 

 re-publication in book form. The first twelve of these 

 were now arranged according to the seasons of the year, 

 "beginning with a winter of activity, and ending with 

 a winter of repose." The Zoological Gardens upon a 

 bleak January day ; a sand-quarry in winter ; a hunt 

 under the bark of decaying tree-stumps in the early 



