INTEODUCTIOK 7 



the embryos of tlie liigliei" Vertebrata, but by-and-by disappear, 

 are structures of tbis kind. Regarded alone they are inexplicable, 

 for they neither lead to the formation of gills at any time, nor are 

 they converted (with the exception of the anterior) into definitive 

 organs of any other kind. Comparison shows us, however, that in a 

 large division of lower Vertebrata these branchial clefts are important 

 organs of respiration; and as we also know Vertebrata (Amphibia), 

 in which the clefts function only for a time as respiratory organs, 

 and close up later on, we are able to comprehend the branchial 

 clefts of reptiles, birds, and mammals, as arrangements obtained by 

 transmission from lower stages, which have lost their primitive 

 function, and remain for a short time only — during foetal life. 



§ 6. 



In the sum of the chai'acters of the organisation, which inherit- 

 ance passes on to an organism, we find, in consequence of what has 

 been already pointed out, a greater or smaller number of an'ange- 

 ments, which pass on into the permanent adult stage of the 

 organism without having any recognisable function in it. Such 

 parts ai'e, as a rule, seen in a more or less atrophied and rudimentary 

 condition, which they often do not acquire until the ontogeny has 

 run its course. In the early stages of the ontogeny they generally 

 agree in completeness with the other arrangements which obtained 

 in the ancestral form from which they are derived. These rudi- 

 mentary organs commence to atrophy the earlier in proportion as 

 they were inherited earlier, in a palseontological sense ; and, as a rule, 

 disappear late when their origin is a relatively late one. The fully- 

 developed form of the rudimentary organs is consequently to be 

 found, in the former case, in widely separated species ; in the latter, 

 on the other hand, in species more closely allied. These organs 

 are valuable objects, since phylogenetic relations can be very 

 generally recognised by their aid. They show, too, how little 

 physiological significance ought to be regarded in a morphological 

 discussion ; for in most of them a function is not to be made out at 

 all, or, if it can be made out, is found to be quite different to the 

 primitive one. 



§ 7. 



Comparative Anatomy forms part of Ontogeny, inasmuch as it 

 treats of the pheenomena of the organisation which appear in the 

 course of the individual development of the animal ; not only in 

 relation to the complete stage of the organism, but in relation also 

 to the definitive arrangements of other organisms. Comparative 



