186 ESSAY ON CLASSIFICATION 



should this be? Simply because the proportions of parts, which con- 

 stitute specific characters, are recognizable before their ultimate 

 structural development, which characterizes genera, is completed. 



It seems to me that these facts are likely to influence the future 

 progress of Zoology in enabling us gradually to unravel more and 

 more distinctly the features which characterize the different subordi- 

 nate gToups of the animal kingdom. The views I have expressed 

 above of the respective value and the prominent characteristics of 

 these different groups have stood so completely the test in this analy- 

 sis of their successive appearance, that I consider this circumstance 

 as adding to the probability of their correctness. 



But this has another very important bearing, to which I have al- 

 ready alluded in the beginning of these remarks. Before Embryology 

 can furnish the means of settling some of the most perplexing prob- 

 lems in Zoology, it is indispensable to ascertain first what are typical, 

 classic, ordinal, family, generic, and specific characters; and as long 

 as it could be supposed that these characters appear necessarily dur- 

 ing the embryonic growth in the order of their subordination, there 

 was no possibility of deriving from embryological monographs that 

 information upon this point, so much needed in Zoology and so sel- 

 dom alluded to by embryologists. Again, without knowing what con- 

 stitutes truly the characters of the groups named above, there is no 

 possibility of finding out the true characters of a genus of which only 

 one species is known, of a family which contains only one genus, etc., 

 and for the same reason no possibility of arriving at congruent results 

 with reference to the natural limitations of genera, families, orders, 

 etc., without which we cannot even begin to build up a permanent 

 classification of the animal kingdom; and, still less, hope to establish 

 a solid basis for a general comparison between the animals now living 

 and those which have peopled the surface of our globe in past geolog- 

 ical ages. 



It is not accidentally I have been led to these investigations, but by 

 necessity. As often as I tried to compare higher or more limited 

 groups of animals of the present period with those of former ages, or 

 early stages of growth of higher living animals with full-grown ones 

 of lower types, I was constantly stopped in my progTess by doubts as 

 to the equality of the standards I was applying, until I made the 

 standards themselves the object of direct and very extensive investiga- 



