In some particular instances the absence of any trace of intermediate forms is especially significant. The tribe of Bats, for instance, is a very singular one. The wings, in which form the fore-limbs are specialized, represent the same elements as our own hands; and other modifications of the same members have produced the paws of cats and dogs, the hoofs of horses and cattle, and the flippers of whales and porpoises,--to mention no others. What countless hosts of the Bat's ancestors must have lived and died while by accumulation of minute differences the primitive generalized limb whence all these diverse forms originated, was being turned into a wing capable of flight. Yet of all these no vestige is to be discovered. "Whenever the remains of bats have been found," says Mr. Mivart,[260] "they have presented the exact type of existing forms." The same, he tells us, holds good of other flying creatures--birds and pterodactyles--(or flying lizards--now wholly extinct). No trace of any of these is forthcoming while their wings were in the making. "Yet had such a slow mode of origin as Darwinians [and genetic evolutionists generally] contend for, operated exclusively in all cases, it is absolutely incredible that bats, birds, and pterodactyles should have left the remains they have, and yet not a single relic be preserved in any one instance of any of these different forms of wing in their incipient and relatively imperfect functional condition!" There are other creatures which stand in solitary isolation, with no fragments of a bridge to connect them with the general body. Such is the rattlesnake's family, whose pedigree, Mr. Mivart declares,[261] we cannot even imagine--"The ancestors of the rattlesnake are beyond our mental vision." But the number of forms [says the same author][262] 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 Icthyosauria 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 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. Professor W. C. Williamson likewise remarks[263] on these _lacunæ_ which persistently occur at crucial points: