260 



HABIT AND INTELLIGENCE. 



[chap. 



"We do not 

 know the 

 data for an 

 answer. 



They are, 



more 



pi-obably, 



records of 



ancestral 



forms. 



Useless 

 organs, 

 and 



purposes for which oiie raode of development is better than 

 another. We can see no purpose in indirect development ; 

 for anything the wisest of us can see, the original struc- 

 tureless germ of every organic species might as well have 

 developed into its mature form by the most direct process ; 

 but we are so utterly ignorant of the conditions of the 

 problem, and any experimental test is so completely out 

 of the question, that it would be unwarrantable pre- 

 sumption in us to deny that there may possibly be as real 

 a purpose for the process of indirect development, as there 

 is for the process of circulation or of respiration. This, I 

 say, may be true, but all that can be said in its favour is, 

 that there is no evidence to disprove it : and it does not 

 appear probable. It appears very improbable, that a 

 change of plan while development is going on should be a 

 necessary law of development ; it appears very much more 

 likely, that the first transitory stage of an animal's develop- 

 ment is a record, or, as it were, a picture, of what the mature 

 form of its remote ancestor was. I believe that the like- 

 ness of the earliest forms of the lower aquatic Invertebrata 

 to Protozoa is a record of their descent from Protozoa; 

 that the resemblance of the larvae of many insects to 

 worms is a record of the descent of insects from worms ; 

 that the resemblance of the branchiae in the larval state of 

 the higher Crustacea to the branchite of the lower Crustacea 

 in their mature state is a record of the descent of the 

 higher Crustacea from the lower ; and that the water- 

 breathing branchiae of the tadpole are a record of the 

 descent of the frog from water-breathers. 



As I have just stated it, I do not say that the argument 

 is conclusive. I think it is comparable to the argument 

 for the descent of all the Vertebrata from a common 

 ancestor, grounded on the homolegical correspondences 

 between their skeletons being so much closer than any 

 community of function requires. But the argument from 

 comparative morphology, which would have been otherwise 

 only a strong, though unverified, presumption, is raised, as 

 I think, into a certainty by the discovery of rudimentary 

 and useless members ; which we cannot believe to have 



