352

ESSAY ON CLASSIFICATION.

curacy and clearness, to present a condensed account ofthose opinions with which he disagreed, in these words:

“Few views of the relations existing in the organicworld have received so much approbation as this: thatthe higher animal forms, in the several stages of the deve-lopment of the individual, from the beginning of its exist-ence to its complete formation, correspond to the perma-nent forms in the animal series, and that the developmentof the several animals follows the same laws as those ofthe entire animal series; that consequently the morehighly organized animal, in its individual development,passes, in all that is essential, through the stages that arepermanent below it, so that the periodical differences ofthe individual may be reduced to the differences of thepermanent animal forms.”

Next, in order to have some standard of comparisonwith his embryological results, he discusses the relativeposition of the different permanent types of animals asfollows:

“ It is especially important that we should distinguishbetween the degree of perfection in the animal structureand the type of organization. The degree of perfectionof the animal structure consists in the greater or lessheterogeneousness of the elementary parts, and the sepa-rate divisions of a complicated apparatus,—in one word,in the greater histological and morphological differentia-tion. The more uniform the whole mass of the body is,the lower the degree of perfection: it is a stage higherwhen nerve and muscle, blood and cellular tissue, aresharply distinguished. In proportion to the differencebetween these parts is the development of the animal lifein its different tendencies; or, to express it more accu-rately, the more the animal life is developed in its several