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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



to withdraw almost instantaneously from those 

 one wishes to shun, to drop suddenly upon cer- 

 tain places, and discover in them some hidden or 

 lovely thing — such are the longings that have 

 agitated many hearts. 



A few of the general features of Nature have 

 necessarily been visible at all times to the eyes 

 of men least disposed to launch into high specu- 

 lations, but nothing has been understood. Those 

 who first conceived the thought of writing the 

 history of animals remained under the control of 

 prevailing ideas, and stopped at appearances, 

 which are enough to content the mind of mere 

 observers ; but the day came when naturalists 

 thought of arranging a sort of inventory of Na- 

 ture. Then, from the necessity of giving a de- 

 scription of each animal such as to cause it to be 

 recognized, grew the attention needed for grasp- 

 ing those peculiarities of conformation common 

 to a greater or less number of species, without 

 paying much regard thenceforward to their kind 

 of life. It began to be perceived that creatures 

 very similar in their characteristic organization 

 might have very different habits and modes of 

 nutrition. Yet with this glimpse of truth men 

 were still far from having a clear conception of 

 the typical forms to which animated beings are 

 bound. Thus it was an event when George Cu- 

 vier, more learned than his predecessors, discov- 

 ered that "there exist four principal forms after 

 which all animals appear to have been modeled." 

 This new light soon gained all imaginable bright- 

 ness from the observations of a professor at St. 

 Petersburg, laboring and meditating with no con- 

 cern for the opinions more or less in vogue. It 

 had occurred to that patient and skillful investi- 

 gator to prove that the characteristics of beings 

 in their embryonic condition confirmed the nat- 

 ural divisions observed by Cuvier, which only 

 required to be corrected as to their boundaries. 

 In another direction, fortunate suggestions, spring- 

 ing up in the early years of our century, direct- 

 ed investigation upon a course peculiarly favor- 

 able to the advance of science. Comparisons 

 methodically made established the certainty that 

 all the representatives of each of the grand zo- 

 ological types are constructed after one and the 

 same fundamental plan, that the differences relate 

 simply to the configuration of the parts, to their 

 degree of development or perfection, to their 

 appropriation to various uses ; an admirable 

 truth, disengaged by slow degrees from obscurity, 

 and afterward defended against ancient errors 

 with a sort of enthusiasm kindled by the feeling 

 of winning a great controversv for the human 



mind. With a success checkered, however, by a 

 few failures, owing to the want as yet of suffi- 

 cient precise knowledge, proofs were constantly 

 accumulated that all vertebrated animals, mam- 

 mals, birds, reptiles, fishes, have the same organs 

 situated in constant relations, and that all ar- 

 ticulated animals, insects, arachnids, Crustacea, 

 are derived from one single primordial plan. As 

 to the vertebrates, this demonstration came in 

 great part from the labors of Geoffroy Saint- 

 Hilaire: as to the articulates, it was made by 

 Savigny. 



Setting out from this point, it became almost 

 easy to understand the reason of the form or the 

 degree of development of different parts in the 

 organism, and to gain the meaning of an altera- 

 tion in the forms. A guide of unerring certainty 

 was given to investigators. Under the influence 

 of the very natural exultation occasioned by the 

 triumphs gained, and yet more by the grand and 

 philosophic idea which had ruled in the investi- 

 gation of parts of like nature, or, to adopt the 

 expression used, of homologous parts in animals 

 so unlike as fishes, reptiles, and mammals, the 

 question was asked, whether unity of composi- 

 tion did not extend throughout the entire animal 

 kingdom. Thus several attempts were made, 

 but not successfully, to assimilate the organs of 

 the insect to those of the vertebrate animal. 

 In these two types the organs do not main- 

 tain either like relations or like positions ; it is 

 only by giving exclusive attention to marks of 

 structure and to the function of the parts that 

 we find it possible to reach a sort of assimila- 

 tion. 



The precise idea of the general constitution 

 of animals having become established, anatomi- 

 cal studies, suitably directed, ceased to find their 

 only object in the configuration of the organs; 

 they tended necessarily toward the interpretation 

 of modifications in the organism, the ascertain- 

 ment of the functions of parts, and the discovery 

 of the mechanism of arrangements. The results 

 gained under the inspiration of such views are 

 vast, and science has enlarged with marvelous 

 rapidity. On the other hand, close study of the 

 tissues has brought to light this important fact, 

 that the primitive elements present the same 

 essential characters in all living beings. By 

 experiments skillfully conducted upon living 

 animals, certain functions of different parts of 

 the organism have been made apparent which 

 doubtless would never have been suspected had 

 other modes of examination been employed. In- 

 vestigations of the successive phases of develop- 



