r 5 6 



THE QUARTERLY REVIEW OF BIOLOGY 



rest in three several organs, viz. the 

 spleen, the liver and the bone marrow. 

 Microscopic examination shows that in 

 these sites the normal vascular endo- 

 thelium is wanting, and that the particles 

 have been ingested by special cells, the 

 KupfFer cells of the liver, the cells of the 

 splenic ellipsoids and other special cells 

 of the marrow. Now, in accordance with 

 the literal terms of Owen's definition it 

 might be stated that these phagocytic 

 vessel-lining cells, which all react in the 

 same way towards particles, are "analo- 

 gous" elements. 



It should be kept in mind, however, 

 that the original purpose of the definition 

 of analogy was not to serve a physiological 

 end, but to prevent confusion in a struc- 

 tural inquiry. The analogous organs of 

 interest to GeofFroy St. Hilaire were, for 

 example, the legs or arms of arthropods, 

 of cephalopods and of vertebrates; the 

 wings of insects, of birds and of bats; 

 the valves of Cirripedia, of brachiopods 

 and of acephalous molluscs; the tail-parts 

 of arthropods and of vertebrates; the 

 tentacles or antennae respectively of 

 gastropod molluscs and of arthropods — 

 in each case structures which to the laity 

 might appear identical, but whose funda- 

 mental structural inequivalence is for the 

 first time revealed by comparative anat- 

 omy. While Owen's definition of 

 analogy was so phrased as not to include 

 any formal reference to structure, the 

 context sufficiently shows that it carried 

 a structural connotation, being designed 

 to anticipate the very blunder that on a 

 superficial view might cause embarrass- 

 ment. Introduced as a complementary 

 term to homology, for use in purely 

 structural inquiries, it was originally 

 aimed at organs which in virtue of similar 

 environment show an external structural 

 correspondence. On its first introduction 

 therefore the word analogy had no 



exclusively functional meaning, con- 

 trary to what some modern writers 

 occasionally assume. Essentially it in- 

 volved a structural connotation, and the 

 real truth is that GeofFroy and Owen, in 

 setting up analogy against homology, 

 were struggling to formulate a distinction 

 between what we should now prefer to 

 call similarity by convergence and simi- 

 larity by descent. 



THE EMBRYOLOGICAL OR GENETIC CRITERION 

 OF HOMOLOGY 



With the advent of Darwinism a 

 curious thing happened in regard to the 

 criteria of homology. Structural like- 

 nesses being seen to depend, not on unity 

 of plan, but on descent from common 

 ancestors, the original geometrical con- 

 ception of homology, which for the sake 

 of distinctness we might now call "posi- 

 tional homology," was allowed quietly 

 to lapse. One might suppose that the 

 term homology, having outlived its raison 

 d'etre, would ultimately have died out. 

 Instead, it adapted itself to its new 

 surroundings and emerged in a new guise 

 and with transformed meaning, which 

 we shall here designate as "embryological 

 homology." The change of meaning of 

 the word, which we have indicated by 

 prefixing to it special adjectives, occurred 

 so easily and so naturally that for a time 

 the matter escaped analytical inquiry. 



Fritz Muller was early in the field with 

 support for Darwin's theory of descent. 

 Muller 's main contention was that during 

 the development of various animals, 

 especially the Crustacea, the embryos pass 

 through successive stages which resemble 

 the presumable line of historical descent 

 of the adults. Haeckel, seizing upon this 

 conception, which he extended with a 

 wealth of illustration to the animal 

 kingdom at large, gave it concise form in 

 his statement of the so-called "biogenetic 



