If I am not mistaken you have intimated something of this sort also in your first edition, but I have not been able to find it lately.

Of course this is a very normal condition of affairs when a series can be followed in this way, beginning with species a, then going through species a b to a b c, then a b d or a c d, and then a d e or simply a e, as it sometimes comes.

Very often the acceleration takes place in two closely connected series, thus: a--ab--abd--ae---ad in which one series goes on very regularly, while another lateral offshoot of a becomes d in the adult.

This is an actual case which can be plainly shown with the specimens in hand, and has been verified in the collections here.

Retardation is entirely Prof.Cope's idea, but I think also easily traceable.