186 GEOLOGICAL DISTRIBUTION. 



to a type once formed, but which has at one or more periods been 

 effaced. If the former, would it be illogical to suppose that some 

 of the numerous varieties or species that may have been evolved 

 during a long lapse of ages from the parental stock should have 

 proved less hardy than it, and should have, therefore, suffered much 

 earlier extinction or modification ? And, if so, what is there that 

 we know that should absolutely prevent the same early extermi- 

 nated forms from being re-evolved ? It will be naturally assumed 

 that the new and the old species, which appear to be so closely 

 connected, are in reality distinct, and this may be true in all cases ; 

 but, if the distinguishing characters separating the two are almost 

 imperceivable, it will require not much stretching of scientific prin- 

 ciples to conceive that at least some of the resulting varieties may 

 be strictly indistinguishable. 



Again, reverting to the case of the pigeons, it seems by no 

 means clear, although such results may be of very rare occurrence, 

 why a fantail, identical in every respect with the common form, 

 may not be produced from a species of pigeon other than the rock- 

 pigeon. It is true, as Darwin states, that newly-formed varieties 

 would be almost sure to inherit from their progenitors certain dis- 

 tinguishing traits or characters ; but, as the formation of a species 

 will depend upon the overbalancing by newly-acquired characters 

 of those that may have been left by inheritance, there seems to be 

 no reason why, through a process of selection and adaptation, such 

 a convergence might not occasionally take place as would unite 

 the ends of very distinct lines of descent. Paleontologists have 

 long held to the opinion that the line of descent which leads up to 

 the horse (genus Equus) in America is different from the similar 

 line of Europe, and if it has been contended that the existing spe- 

 cies, Equus caballus, was not in itself a product of the American 

 line, but a modern European importation, its recent discovery in 

 the Post-Pliocene deposits of this country proves the erroneousness 

 of such an assumption. If, therefore, we are permitted to assume 

 that two distinct lines of descent, the one passing through Eohip- 

 pus, Orohippus, Mesohippus, Miohippus, Protohippus, and Plio- 

 hippus, and the other through False otherium, Auchitherium, and 

 Hipparion, lead up to an equivalent genus, the genus horse or 

 Equus, would it be unreasonable to suppose that, with a continu- 

 ance of the process of evolution, even the same species may be 



