214 LEISURE-TIME STUDIES. 



produce each a single egg, which developed into a single 

 salpa ; and the latter, conversely, produced each a long 

 u chain " of individuals, the one variety, in fact, reproducing 

 the other. The apparently mutual development of the zoo- 

 phyte and the jelly-fish, and of the chain and single salpa is, 

 however, explicable, as far as its exact nature goes, on other 

 grounds than those on which the naturalists of former years 

 accounted for the phenomena. The jelly-fish is not a distinct 

 animal from the zoophyte, but merely one of its modified 



FIG. 31. Single salpa. 



buds, produced, like the other parts of the animal tree, by a 

 process of budding, and destined for a special end, that of 

 the development of eggs. The latter illustrate the law of 

 heredity because they are to be regarded as having been 

 essentially and truly produced by the zoophyte, into the 

 form of which each egg directly develops. And similarly 

 with the salpje. The chain-salpa may be regarded as corres- 

 ponding to the zoophyte, each individual of the chain pro- 

 ducing an egg, which develops again into a chain-salpa, 

 through the medium of the single and unconnected form. 



To a still greater extent in insects and some crustaceans 

 such as barnacles, etc. may the process of development be 

 complicated and extended. The egg of the butterfly gives 

 origin, not to the aerial winged insect, but to the mundane 

 caterpillar (Fig. 32), which, after passing an existence devoted 

 solely to the work of nourishing its body, envelops that body 

 in a cocoon and becomes the chrysalis ; finally appearing 

 from this latter investment as the winged and mature form. 

 In the case of all insects which, like the butterfly, pass 



