184 ESSAY ON CLASSIFICATION 



it presents such peculiarities as distinguish its type. It cannot there- 

 fore be said that any animal passes through phases of development, 

 which are not included within the limits of its own type; no Verte- 

 brate is or resembles, at any time, an Articulate, no Articulate a 

 Mollusk, no Mollusk a Radiate, and vice versa. Whatever correlations 

 between the young of higher animals and the perfect condition of 

 inferior ones may be traced, they are always limited to representa- 

 tives of the same great types; for instance. Mammalia and Birds in 

 their earlier development exhibit certain features of the lower classes 

 of Vertebrates, such as the Reptiles or Fishes; Insects recall the 

 Worms in some of their earlier stages of growth, etc., but even this 

 requires qualifications to which we shall have to refer hereafter. 

 However, this much is already evident, that no higher animal passes 

 through phases of development recalling all the lower types of the 

 animal kingdom, but only such as belong to its own branch.-^ What 

 has been said of the infusorial character of young embryos of Worms, 

 Mollusks, and Radiates can no longer stand before a serious criticism, 

 because, in the first place, the animals generally called Infusoria 

 cannot themselves be considered as a natural class; and, in the second 

 place, those to which a reference is made in this connection are them- 

 selves free-moving embryos.^" 



With the progress of growth and in proportion as the type of an 

 animal becomes more distinctly marked in its embryonic state, the 

 plan of structure appears also more distinctly in the peculiarities of 

 that structure, that is to say, in the ways in which and the means by 

 which the plan, only faintly indicated at first, is to be carried out and 

 become prominent, and by this the class character is pointed out. 

 For instance, a wormlike insect larva will already show by its tracheal 

 that it is to be an Insect and not to remain a Worm, as it at first ap- 

 pears to be; but the complications of that special structure, upon 

 which the orders of the class of Insects are based, do not yet appear; 

 this is perfected only at a late period in the embryonic life. At this 

 stage we frequently notice already a remarkable advance of the fea- 

 tures characteristic of the families over those characteristic of the 



^ [The preceding statements represent Agassiz's unwillingness to accept the recapitu- 

 lation concept or biogenetic law in its most radical form. His criticism stemmed from 

 the conviction that the notion of a complete recapitulation of all racial history from 

 lower to higher forms came very close to a theory of organic evolution.] 



'"See above. Chap. I, Sect, xviii. 



