axis in a manãs skull, but what is a straight line in the one has become a certain definite curve in the other.
Mr Heilmann tells me that he has tried, but without success, to obtain a transitional series between the human skull and some prehuman, anthropoid type, which series (as in the case of the Equidae) should be found to contain other known types in direct linear sequence. It appears impossible, however, to obtain such a series, or to pass by successive and continuous gradations through such forms as Mesopithecus, Pithecanthropus, Homo neanderthalensis, and the lower or higher races of modern man. The failure is not the fault of our method. It merely indicates that no one straight line of descent, or of consecutive transformation, exists; but on the contrary, that among human and anthropoid types, recent and extinct, we have to do with a complex problem of divergent, rather than of continuous, variation. And in like manner, easy as it is to correlate the baboonãs and chimpanzeeãs skulls severally with that of man, and easy as it is to see that the chimpanzeeãs skull is much nearer to the human type than is the baboonãs, it is also not difficult to perceive that the series is not, {773} strictly speaking, continuous, and that neither of our two apes lies precisely on the same direct line or sequence of deformation by which we may hypothetically connect the other with man.
As a final illustration I have drawn the outline of a dogãs skull (Fig. 408), and inscribed it in a network comparable with the Cartesian network of the human skull in Fig. 404. Here we attempt to bridge over a wider gulf than we have crossed in any of our former comparisons. But, nevertheless, it is obvious that our method still holds good, in spite of the fact that there are various specific differences, such as the open or closed orbit, etc., which have to be separately described and