Such combinations of diverse characters are not limited to animals which are extinct. There were not wanting scientific men who regarded the Platypus of Australia, when first sent to Europe, as an ingenious example of Eastern skill, in which an animal had been compounded artificially by blending the beak of a Bird with the body of a Mammal. Fuller knowledge of that remarkable animal has continuously intensified wonder at its combination of Mammal, Bird, and Reptile in a single animal. It has broken down the theoretical divisions between the higher Vertebrata, demonstrating that a Mammal may lay eggs like a Reptile or Bird, that the skull may include the reptilian characters of the malar arch and pre-frontal and post-frontal bones, otherwise unknown in Mammals and Birds. The groups of Mammals, Birds, and Reptiles now surviving on the earth prove to be less sharply defined from each other when the living and extinct types are considered together. But in Pterodactyles, Mammal Bird and Reptile lose their identity, as three colours would do when unequally mixed together. This mingling of characteristics of different animals is not to be attributed to interbreeding, but is the converse of the combination of characters found in hybrid animals. It is no exaggeration to say that there is a sense in which Mammal, Bird, Reptile, and the distinctive structures of the Ornithosaur, have simultaneously developed from one egg, in the body of one animal. The differences between those vertebrate types of animals consist chiefly in the way in which their organisation is modified, by one strain of characters being eliminated so that another becomes predominant, while a distinctive set of structures is elaborated in each class of animals. The earlier geological history of the higher Vertebrata is very imperfectly known, but the evidence tends to the inference that the older representatives of the several classes approximate to each other more closely than do their surviving representatives, so that in still earlier ages of time the distinction between them had not become recognisable. The relation of the great groups of animals to each other, among Vertebrata, is essentially a parallel relation, like the colours of the solar spectrum, or the parallel digits of the hand. It was natural, when only the surviving life on the earth was known, to imagine that animals were connected in a continuous chain by successive descent, but Mammals have given no evidence of approximation to Birds; and Birds discover no evidence that their ancestors were Reptiles, in the sense in which that word is used to define animals which now exist on the earth. When the variation which animals attain in their maturity and exhibit in development from the egg was first realised, it was imagined that Nature, by slow summing up and accumulation of differences which were observed, would so modify one animal type that it would pass into another. There is little evidence to