174 ESSAY ON CLASSIFICATION. Tart I. 



phases of development, which are not inclnded within the hmits of its own type ; 

 no Vertebrate is, or resembles, at any time an Articulate, no Articulate a MoUusk, 

 no MoUusk a Eadiate, 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 representatives 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 Keptiles or Fishes; Insects recall the Worms in some 

 of their earlier stages of growtli, etc., but even this requires qualifications to 

 which we shall have to refer hereafter. However, thus 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 themselves 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 wonnlike insect larva will already show, by its tracheae, 

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

 able advance of the features characteristic of the fixmilies over those characteristic of 

 the order; for instance, young Hemiptera, young Orthoptera may safely be referred 

 to their respective families, from the characteristics they exhibit before they show 

 those pecuharities which characterize them as Hemijjtera or as Orthoptera; young 

 Fishes may be known as members of their respective families before the charac- 

 ters of their orders are apparent, etc. 



It is very obvious why this should be so. With the progress of the develop- 

 ment of the structure, the general form is gradually sketched out, and it has 

 already reached many of its most distinctive features, before all the complications 

 of the structure which characterize the orders have become apparent ; and as form 

 characterizes essentially the families, we see here the reason why the family type 



' See above, Chap. I., Sect. 18, p. 75. 



