i6o 



THE QUARTERLY REVIEW OF BIOLOGY 



material to begin with." For establish- 

 ment of homoplasy, according to the 

 definition, both conditions have to be 

 satisfied. Condition (i) is phrased in 

 sufficiently general terms to include ex- 

 ternal as well as internal environment. 

 The main question that arises is the 

 precise degree of restriction imposed by 

 condition (z). In a reply to H. F. Osborn 

 (1907) he says, "Can you imagine cases 

 of convergence or parallelism which are 

 not covered by the definition I gave of 

 homoplasy? What organs are parallel 

 in any two animals and yet have no 

 likeness at all — even the most general — 

 in their material?" 



From these sentences it would appear 

 that Lankester might have been willing 

 to omit condition (z) from his definition. 

 It must be kept in mind that in his original 

 paper he did more than define homoplasy; 

 he committed himself to a statement or 

 proposition as to the action of similar 

 environmental forces on two separate 

 things, and, for the purpose of this propo- 

 sition, condition (z) may have been 

 necessary. When, however, close struc- 

 tural likeness is seen to arise between two 

 very diverse things — likeness which is 

 obviously correlated with, or rather 

 dependent upon, identity of environment 

 — it should be quite permissible to desig- 

 nate the resemblance as homoplasy. The 

 various instances of analogy are just of 

 this kind, e.g., the wings of bats, of birds 

 and of insects; the eyes of Pecten, of 

 cephalopods and of vertebrates; the 

 otocysts of various invertebrates and 

 their labyrinthine equivalents in verte- 

 brates; the division of the body into 

 leaves, stem and roots in the case of the 

 higher plants (diploid generation) and 

 of mosses (haploid generation); the hyphal 

 form of parasites, such as the Fungi 

 among lower plants, Rafflesia among 

 higher plants, Sacculina among animals; 



the hold-fast organs (haptera) of sea- 

 weeds and green algae, perhaps the 

 adhesive roots of ivy and adhesive stems 

 of Ampelopsis, the adhesive base of certain 

 unicellular animals (e.g., Vorticella), of 

 sponges, of zoophytes, of sea anemones, 

 of crinoids, of Cirripedia and of sessile 

 tunicates. If this interpretation of Lan- 

 kester's meaning be correct, then, as 

 Osborn says in an amended statement, 

 "Lankester's homoplasy is equivalent to 

 analogous evolution, to parallelism, or 

 convergence. 



EXPERIMENTAL CONTRIBUTIONS TO 

 HOMOPLASY 



Just as Owen based the conception of 

 analogy on morphological evidence, so 

 it was by purely morphological testimony 

 that Lankester was led to institute the 

 idea of homoplasy. Of recent years 

 physiology and experimental embryology 

 have begun — in some cases almost unwit- 

 tingly — to furnish evidence touching upon 

 the question of homoplasy. With the 

 help of some illustrative cases we may 

 briefly indicate the significance of these 

 experimental contributions to the subject. 



Examples. It is well known that 

 normal development of (1) the accessory 

 sexual organs (e.g., the vagina, uterus, 

 Fallopian tubes and mammary glands of 

 the female, the vesiculae seminales and 

 prostate gland of the male) and (z) of the 

 so-called [secondary sexual characters of 

 mammals, is dependent upon the [presence 

 of the gonads. When the ovaries or the 

 testes of a vertebrate are removed in 

 youth, the growth and differentiation of 

 the accessory sexual organs is faulty and 

 the secondary sexual characters are absent. 

 By grafting into the castrated animal a 

 gonad taken from another animal of 

 similar species and sex, the defective 

 organs or characters are induced to resume 

 development. Experiment has shown 



