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THE QUARTERLY REVIEW OF BIOLOGY 



and that this energy is used simultaneously 

 for its support and for its forward progres- 

 sion, in virtue of the reaction or inertia of 

 the air. So defined, flight would include 

 the flight of a bird or of an insect, of a 

 flying machine, of a rocket and of an 

 aerial torpedo, but not of a flying fish, of 

 a pterodactyl or of a flying phalanger. 

 To one whose pursuits have lain chiefly in 

 the field of comparative anatomy such 

 analysis may appear fussy and beside the 

 point, particularly in its invocation of 

 energy considerations, and that to the 

 exclusion of the wing itself. To the 

 student of function this mode of procedure 

 is a sine qua non; it is his comparative 

 method; in which respect our cited case is 

 merely an example of a general and 

 invariable mode of attack. With advance 

 of time, too, the elaboration of specifica- 

 tion tends to increase, and even our 

 example involves elements that are novel 

 since the time of Cuvier and Geoffroy. 

 Similar analysis is involved in terms like 

 walking, running, swimming, breathing, 

 feeding, excreting, etc. When we say 

 that a system of organs, or even a partic- 

 ular part, subserves a certain use or func- 

 tion, it is necessary, in order to avoid 

 confusion, to be scrupulously explicit. 

 As a very simple illustration let one con- 

 sider the fundamental difference between 

 feeding of a Paramecium, which ingests 

 food particles bodily into its protoplasm, 

 and the corresponding process in higher 

 animals, which first reduce their food to a 

 soluble condition before absorption. 



The two meanings of "function." An- 

 other, and more vital, complication is that 

 the word function is ordinarily employed 

 in two quite distinct senses: (i) to indi- 

 cate the general use or application of an 

 organ, as in the phrase "to subserve a 

 function"' — the cases already cited come 

 under this category; (z) to cover the 

 individual events or interior processes 



which go on in its constituent cells, i.e., 

 the essential or intrinsic physiological 

 mechanism of the part. Modern physi- 

 ology has concerned itself particularly 

 with function in the second sense, which 

 we may here for convenience call "Func- 

 tion B," whereas Geoffroy St. Hilaire and 

 Owen, in their institution of the concep- 

 tion of analogy, were thinking only of the 

 first, which we may call by the name of 

 "Function A." Thus it is that a physi- 

 ologist of the present day, suddenly 

 confronted with Owen's definition of hom- 

 ology and of analogy, is at a loss to under- 

 stand from the examples cited by way of 

 illustration wherein the averred distinc- 

 tion lies. He instinctively looks for a 

 difference in essential mechanism, let us 

 say, between legs, wings and fins, and 

 finds none. To him all these appendages 

 are simply pegs or bone-linkages moved in 

 an adequately coordinated fashion by; 

 muscles under the domination of thei 

 central nervous system — cf. Keith Lucas. 

 Similar use but different intrinsic mech- 

 anism. On the other hand — and this 

 involves a third complication — it has- 

 been found that organs which fulfil 

 equivalent requirements so far as the 

 needs of the animal are concerned, may; 

 differ in intrinsic physiological mechanism. | 

 Thus, the heart of a mammal and the 

 heart of the king-crab Limulus perform in 

 each case the office of pumping the blood. I 

 The muscle of the mammalian heart, 

 however, exhibits a marked refractory 

 phase after each beat, cannot be held in 

 continued contraction by serial stimula- 

 tion and has inherent rhythmic power, 

 whereas the muscle of the heart of Limulus 

 has no enduring refractory phase, can by 

 a series of stimuli be maintained in steady 

 contraction and ceases to beat when 

 separated from the central nervous system 

 (Carlson). To Geoffroy St. Hilaire and 

 to Owen these hearts, which prove on 



