a history of science-4-第20章
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th their blood exposed beneath the thin surface of their leaves； since when pure air is thus applied it is probable that it can be more readily absorbed。 Hence； in the curious experiments of Dr。 Priestley and Mr。 Ingenhouz； some plants purified less air than othersthat is； they perspired less in the sunshine； and Mr。 Scheele found that by putting peas into water which about half covered them they converted the vital air into fixed air； or carbonic…acid gas； in the same manner as in animal respiration。
〃7。 The circulation in the lungs or leaves of plants is very similar to that of fish。 In fish the blood； after having passed through their gills； does not return to the heart as from the lungs of air…breathing animals； but the pulmonary vein taking the structure of an artery after having received the blood from the gills； which there gains a more florid color； distributes it to the other parts of their bodies。 The same structure occurs in the livers of fish； whence we see in those animals two circulations independent of the power of the heartviz。； that beginning at the termination of the veins of the gills and branching through the muscles； and that which passes through the liver； both which are carried on by the action of those respective arteries and veins。〃＇6＇
Darwin is here a trifle fanciful in forcing the analogy between plants and animals。 The circulatory system of plants is really not quite so elaborately comparable to that of fishes as he supposed。 But the all…important idea of the uniformity underlying the seeming diversity of Nature is here exemplified； as elsewhere in the writings of Erasmus Darwin； and； more specifically； a clear grasp of the essentials of the function of respiration is fully demonstrated。
ZOOLOGY AT THE CLOSE OF THE EIGHTEENTH CENTURY
Several causes conspired to make exploration all the fashion during the closing epoch of the eighteenth century。 New aid to the navigator had been furnished by the perfected compass and quadrant； and by the invention of the chronometer； medical science had banished scurvy； which hitherto had been a perpetual menace to the voyager； and； above all； the restless spirit of the age impelled the venturesome to seek novelty in fields altogether new。 Some started for the pole； others tried for a northeast or northwest passage to India； yet others sought the great fictitious antarctic continent told of by tradition。 All these of course failed of their immediate purpose； but they added much to the world's store of knowledge and its fund of travellers' tales。
Among all these tales none was more remarkable than those which told of strange living creatures found in antipodal lands。 And here； as did not happen in every field； the narratives were often substantiated by the exhibition of specimens that admitted no question。 Many a company of explorers returned more or less laden with such trophies from the animal and vegetable kingdoms； to the mingled astonishment； delight； and bewilderment of the closet naturalists。 The followers of Linnaeus in the 〃golden age of natural history；〃 a few decades before； had increased the number of known species of fishes to about four hundred； of birds to one thousand； of insects to three thousand； and of plants to ten thousand。 But now these sudden accessions from new territories doubled the figure for plants； tripled it for fish and birds； and brought the number of described insects above twenty thousand。 Naturally enough； this wealth of new material was sorely puzzling to the classifiers。 The more discerning began to see that the artificial system of Linnaeus； wonderful and useful as it had been； must be advanced upon before the new material could be satisfactorily disposed of。 The way to a more natural system； based on less arbitrary signs； had been pointed out by Jussieu in botany； but the zoologists were not prepared to make headway towards such a system until they should gain a wider understanding of the organisms with which they had to deal through comprehensive studies of anatomy。 Such studies of individual forms in their relations to the entire scale of organic beings were pursued in these last decades of the century； but though two or three most important generalizations were achieved （notably Kaspar Wolff's conception of the cell as the basis of organic life； and Goethe's all…important doctrine of metamorphosis of parts）； yet； as a whole； the work of the anatomists of the period was germinative rather than fruit…bearing。 Bichat's volumes； telling of the recognition of the fundamental tissues of the body； did not begin to appear till the last year of the century。 The announcement by Cuvier of the doctrine of correlation of parts bears the same date； but in general the studies of this great naturalist； which in due time were to stamp him as the successor of Linnaeus； were as yet only fairly begun。
V。 ANATOMY AND PHYSIOLOGY IN THE NINETEENTH CENTURY
CUVIER AND THE CORRELATION OF PARTS
We have seen that the focal points of the physiological world towards the close of the eighteenth century were Italy and England； but when Spallanzani and Hunter passed away the scene shifted to France。 The time was peculiarly propitious； as the recent advances in many lines of science had brought fresh data for the student of animal life which were in need of classification； and； as several minds capable of such a task were in the field； it was natural that great generalizations should have come to be quite the fashion。 Thus it was that Cuvier came forward with a brand…new classification of the animal kingdom； establishing four great types of being； which he called vertebrates； mollusks； articulates； and radiates。 Lamarck had shortly before established the broad distinction between animals with and those without a backbone； Cuvier's Classification divided the latterthe invertebratesinto three minor groups。 And this division； familiar ever since to all students of zoology； has only in very recent years been supplanted； and then not by revolution； but by a further division； which the elaborate recent studies of lower forms of life seemed to make desirable。
In the course of those studies of comparative anatomy which led to his new classification； Cuvier's attention was called constantly to the peculiar co…ordination of parts in each individual organism。 Thus an animal with sharp talons for catching living preyas a member of the cat tribehas also sharp teeth； adapted for tearing up the flesh of its victim； and a particular type of stomach； quite different from that of herbivorous creatures。 This adaptation of all the parts of the animal to one another extends to the most diverse parts of the organism； and enables the skilled anatomist； from the observation of a single typical part； to draw inferences as to the structure of the entire animala fact which was of vast aid to Cuvier in his studies of paleontology。 It did not enable Cuvier； nor does it enable any one else； to reconstruct fully the extinct animal from observation of a single bone； as has sometimes been asserted； but what it really does establish； in the hands of an expert； is sufficiently astonishing。
〃While the study of the fossil remains of the greater quadrupeds is more satisfactory；〃 he writes； 〃by the clear results which it affords； than that of the remains of other animals found in a fossil state； it is also complicated with greater and more numerous difficulties。 Fossil shells are usually found quite entire； and retaining all the characters requisite for comparing them with the specimens contained in collections of natural history； or represented in the works of naturalists。 Even the skeletons of fishes are found more or less entire； so that the general forms of their bodies can； for the most part； be ascertained； and usually； at least； their generic and specific characters are determinable； as these are mostly drawn from their solid parts。 In quadrupeds； on the contrary； even when their entire skeletons are found； there is great difficulty in discovering their distinguishing characters； as these are chiefly founded upon their hairs and colors and other marks which have disappeared previous to their incrustation。 It is also very rare to find any fossil skeletons
