But the number of forms represented by many individuals, yet by _no transitional ones_, is so great that only two or three can be selected as examples. Thus those remarkable fossil reptiles, the Ichthyosauria and Plesiosauria, extended, through the secondary period, probably over the greater part of the globe. Yet no single transitional form has yet been met with in spite of the multitudinous individuals preserved. Again, with their modern representatives the Cetacea, one or two aberrant forms alone {133} have been found, but no series of transitional ones indicating minutely the line of descent. This group, the whales, is a very marked one, and it is curious, on Darwinian principles, that so few instances tending to indicate its mode of origin should have presented themselves. Here, as in the bats, we might surely expect that some relics of unquestionably incipient stages of its development would have been left. [Illustration: SKELETON OF A PLESIOSAURUS.] The singular order Chelonia, including the tortoises, turtles, and terrapins (or fresh-water tortoises), is another instance of an extreme form without any, as yet known, transitional stages. Another group may be finally mentioned, viz. the frogs and toads, anourous Batrachians, of which we have at present no relic of any kind linking them on to the Eft group on the one hand, or to reptiles on the other. The only instance in which an approach towards a series of nearly related forms has been obtained is the existing horse, its predecessor Hipparion and other extinct forms. But even here there is no proof whatever of modification by minute and infinitesimal steps; _a fortiori_ no approach to a proof of modification by "Natural Selection," acting upon indefinite fortuitous variations. On the contrary, the series is an admirable example of successive modification in one special direction along one beneficial line, and the teleologist must here be allowed to consider that one {134} motive of this modification (among probably an indefinite number of motives inconceivable to us) was the relationship in which the horse was to stand to the human inhabitants of this planet. These extinct forms, as Professor Owen, remarks,[129] "differ from each other in a greater degree than do the horse, zebra, and ass," which are not only good _zoological_ species as to form, but are species _physiologically_, _i.e._ they cannot produce a race of hybrids fertile _inter se_.