) 



268 



HABIT AND INTELLIGENCE. 



[chap. 



Likeness 

 of larval 

 form 

 proves 

 affinity : 

 but not 

 the 

 converse. 



Insects. 



Beetles. 



Land sala- 

 mander. 



Cnistacea. 



when properly understood, will, I believe, be found not to 

 disprove but to confirm the principles by which I have 

 endeavoured to explain the laws ; and at the same time to 

 throw an important light on the origin of species. 



When two organisms are developed from similar em- 

 bryonic or larval forms, we infer that they are fundamentally 

 alike ; as in the case of the-Cirrhipedes, which, as mentioned 

 above, are now regarded as closely allied to the Crustacea, 

 not from any likeness in their mature forms, but because 

 they are developed from larvae which are crustacean. It 

 is indeed an axiom, that forms which are alike in their 

 earliest stage of development are fundamentally alike, and 

 are to be classed together. But the converse does not hold : 

 organisms may be really allied, and yet may be developed 

 out of very different larval forms. I^o one doubts, for 

 instance, that the true or hexapod insects constitute a 

 perfectly natural class ; that is to say, a class whereof all 

 the members have real and decided affinities to each other. 

 Most of them leave the egg in a worm-like form ; but " in 

 some few cases, as that of Aphis, if we look to the ad- 

 mirable drawings by Professor Huxley of the development 

 of this insect, we see hardly any trace of the vermiform 

 stage." ^ And beetles constitute a perfectly natural order, 

 yet some beetles are much more directly developed, and 

 undergo much less metamorphosis, than others. Quite 

 as remarkable is the case of the land salamander, 

 which, unlike most of the higher Batrachians, does not 

 pass through the tadpole stage, but leaves the egg as an 

 air-breathing animal ; yet we cannot doubt its affinity 

 with the other salamanders, which begin their life as 

 water-breathing tadpoles. Similar facts are observed 

 among the Crustacea. "Fritz Midler has lately made 

 the remarkable discovery that certain shrimp-like Crus- 

 taceans, allied to Penoeus, first appear under the simple 

 naupHus form, and passing through two oT more zoea 

 stages, and through the mysis stage, fijaally acquire their 

 mature structure ; now in the whole enormous malacostra- 

 can class, to which these crustaceans belong, no other 



^ Darwin's Origin of Species, p. 523. 



