172 NATURE'S TEACHINGS. 



datory Tachinae, which are always hovering over such nests, and 

 trying to deposit eggs therein. For many years I have been 

 in the habit of receiving letters from novices in entomology, 

 wanting to know whether I am aware that the common House- 

 fly is in the habit of acting as a parasite. Of course, the 

 writer has mistaken the Tachina for a house-fly, but I cannot 

 regret the fact that some one has really begun to observe 

 Nature, and not only to read books. 



DOORS AND HINGES. 



HAVING seen that both in Nature and Art the entrances to 

 dwellings are guarded by tunnel-like approaches, we come 

 naturally to another mode of guarding the entrance, namely, 

 by a door moving on hinges. As to the multitudinous examples 

 of doors and hinges in modern civilisation, we need hardly dis- 

 cuss them, except to show the exact analogies which occur in 

 Art and Nature. 



Doors moving on hinges are very plentiful in Nature, even 

 where we should least expect them. Take, for example, an egg, 

 especially the egg of an insect, and we shall see that it is just- 

 about the last object in which we should expect to find a hinged 

 door. Yet, if the reader will refer to the illustration on page 7, 

 he will see that the tiny eggs of the common Gnat, numerous 

 as they may be, are each furnished with a door which opens 

 as soon as the inmate is hatched, and allows the little larva 

 to escape into the water. 



Another still more remarkable instance of a hinged door in 

 an egg is to be found in one of the Rotifers, or Wheel- Animal- 

 cules, so called because they possess an apparatus of movable 

 cilia, which, when set in motion, looks exactly like a wheel 

 running round and round. As the full-grown creature is barely 

 one thirty-sixth of an inch in total length, the structure of its 

 eggs must be infinitesimally beyond the range of human vision. 



Yet, just as the telescope sets at partial defiance the vast 

 spaces that intervene between our earth and her sister planets, 

 so the microscope performs a similar tusk in the infinitesimally 

 minute. And, under the all-revealing lens of the microscope, 

 the little egg of the Brachionus, though absolutely invisible to 

 the unaided eye, yields up its secrets, 



