NATURAL SCIENCES OF PHILADELPHIA. 275 



Those naturalists (who are nbt a few) who will be disposed on this account 

 to deny generic rank to Ilemioplites, will have, on the same grounds, to unite 

 each succeeding step till they embrace the " series," and no doubt at the same 

 time belie a considerable amount of their own work already done. 



10. The Coreopsis discoideaT. and G , var. anomala Gray, is much more 

 nearly allied to Bidens frondosa than to other species of its own genus, 

 and the latter is nearer to it than to other species ©f Bidens. It differs chiefly, 

 if not altogether, in the generic character : the barbs of the achenia are di- 

 rected upwards ; those of the Bidens downwards. 



From these and many other such instances it may be derived : That the 

 nearest species of adjacent genera are more nearly allied in specific characters than 

 the most diverse species of the same genus. 



11. While Taxodium d i s t i c h urn and Glyptostrobus europajus, conifers of 

 North America and of Eastern Asia, respectively, are readily distinguished by 

 generic peculiarities of their cones ; in specific characters they appear to be 

 identical.* 



Confirmatorj' of this proposition is the statement of Parker :f " In tracing 

 out the almost infinite varieties of the modifications of any one specific type 

 of shelled Rhizopod, my friend, Prof. Rupert Jones and I found that li/<e va- 

 rieties of distinct species are much nearer in shape and appearance than unlike va- 

 rieties of the same essential species." (It is not unlikely that species should here 

 be read genus and variety species, though the latter may not fulfil the require- 

 ments in regard to distinctiveness observed among higher animals. In types 

 like the Rhizopod, forms of this grade may not be really differentiated. Their 

 enormous geographical range would suggest this, if nothing else.) 



Objection. — A class of objectors to the preceding explanation of the rela- 

 tions in question, will ascribe them to hybridization. They have already done 

 so to considerable extent among the Teleosts, (see the writings of Von Siebold, 

 Steindachner and Giiniher). That hybrids exist in nature will be denied by 

 none, but that they are usual or abundant is not a probable condition of a 

 creation regulated by such order as ours is. The tendency to modify in given 

 lines of generic series, if admitted, will account for many of the cases regard- 

 ed as hybrids by the above authors, for it is to be remarked in many cases how 

 the generic characters are strikingly affected, and are chiefly us(^ in guessing 

 at the parentage. This is among Cyprinidaa so much the case that their is 

 scarce an example of a hybrid between two species of the same genus brought 

 forward, but often between species of different genera. 



If any two forms should hybridize freely, the circumstance should prevent 

 their recognition as distinct species. 



7 Ascertained cases of transition. 



This naturally suggests that in accordance with the theory of acceleration 

 and retardation, a transition can take place in the life history of species. Have 

 we any means of proving this suspicion ? 



1. The genus Ameiva (Saurians of South America) has been composed of 

 species of moderate size furnished with acutely tricuspid teeth. Teius, on the 

 other hand, embraces very large species with the molars obtusely rounded 

 and of the grinding type. These genera are generally held to be well founded 

 at present. 1 find, however, that in Ameiva p 1 e i i, which is the largest species 

 of the genus, that in adults the greater part of the maxillary and mandibular 

 teeth lose their cusps, become rounded, then obtuse, and finally like those of 

 Teius. While young, they are true Ameivs. Strangely enough the A. p 1 ei i, 

 from Porto Rico, acquires but three such obtuse teeth when of the size of the 

 other (St. Croix) forms. In youth the teeth of all are as in other Ameiva;. 

 Here is a case of transition from one genus to another in the same species. 



* See Meehan, Proe. Amer. Ass. Adv. Soi. 1808. Newberry, Ann. Lye, N. Y., 18G8. 

 t Transae. Zool. Soc, London, ! SW, 151. 



1868.] 



