288 PROCEEDINGS OP THE ACADEMY OF 



as two great homologous series, with their contained heterologous genera. 

 As it is, these heterologous terms or genera are evidently so nearly allied 

 that Agassiz, in the Poissons Fossiles, has thought it best to arrange the 

 latter together, thus instituting a system transverse, as it were, to the other. 

 This may be necessary, since Kolliker points out transitional forms, and per- 

 haps certain types may have begun to abandon the heterocercal form near 

 the period of reproduction, producing offspring somewhat protean in charac- 

 ter, preparatory to an earlier appearance and consequent permanence of the 

 homocercal type. This is to be derived from the history of the metamorpho- 

 sis of Amblystoma. 



In the same manner the development of the convolutions of the brain does 

 not define groups of the highest rank, since it progresses chieflj' during the 

 later periods of embryonic life, and is therefore a " developmental character." 

 Owen has endeavored to distinguish the primary divisions of Mammalia by 

 the character of these convolutions, whereas they really define only the sub- 

 groups of the orders. For we have Lissencephalous (smooth-brained) mon- 

 keys, — certain lemurs, — and smooth-brained Ruminants, — i. e., the extinct 

 Brachyodon and Anoplotherium, according to Lartet and Gratiolet. (The 

 lowest types of the existing smooth-brained Mammalia, including especially 

 those with no or rudimental corpus callosum, the Marsupials, are also dis- 

 tinguished by the non-developement of the deciduous teeth* (excepting one 

 premolar). If now through some topographical change the whole series of 

 Mammalia between the smooth-brained and convolute-brained were lost to us, 

 as by the elevation of a region, and the absence of favorable localities or 

 bodies of water for the preservation of their remains, we would have to 

 study two homologous groups, with the heterologous terms of each corres- 

 jjonding with each other, as do now the genera of the Clamatores and 

 Oscines, of the Arcifera and Raniformia, etc. 



In the same way the characters defining Implacental Mammalia will be 

 found transitional in some type, and this great series, homologous with the 

 Placentals, will have to be placed in closer connection, in its genera, with the 

 series of the latter, with genera of the same, perhaps now extinct. 



7. Of mimetic analogy. 



It has been often remarked that the animals of the Equatorial Ethiopian 

 region wereTery generally of smoky and black colors. This is remarkably 

 the case, and the peculiarity of the genus Homo in this respect is shared by 

 birds, reptiles and fishes in a remarkable degree. This cannot be traced to 

 the effect of torrid climate, for the same latitudes in India and the Malaysian 

 Archipelago, and in South America, do not produce such colors. 



The similarity in color of desert types has also been remarked. The grey 

 sand-hue so well adapted for concealment is universal, with few variations, 

 in the reptiles of the Tartar and Arabian deserts, the great Sahaja, and the 

 sands of Arizona and California. There is also a tendency to produce spiny 

 forms in such places; witness the Stellios and Uromastix and Cerastes of the 

 Sahara, the Phrynosomas and horned rattlesnake of south-western America. 

 The vegetation of every order, we are also informed, is in these situations 

 extremely liable to produce spines and thorns. 



The serpents of the Neotropical Region furnish remarkable illustrations of 

 mimetic analogy. All the species of the genera Elaps, Pliocercus, Oxyrrho- 

 pus, Erythrolamprus, and many of those of Ophibolus and Rhabdosoma are 

 ornamented with black and yellow rings on a crimson ground. The species of 

 all these genera are harmless, except in the case of Elaps, which is venomous. 

 We may give for this genus, as the most varied, the following range of varia- 

 tion in coloration : 



* This I have inadvertently alluded to (p. ) as the non-development of the permanent 

 series; the homology of the" dental system of Marsupials appears, however, to be with the 

 latter, and not with the milk: series. See Flower, Trans. Roy. See. 1867. 



[Oct. 



