The title of this book may sound a little outr to some biologists to-day, but cannot be misunderstood in the course of a few years, when the breeding experiments of Prof. Ewart will be more generally known to zoological science. Our readers will remember a paper "On Zebra-Horse Hybrids," which appeared in these pages last year, and which in the 'Penycuik Experiments' is reproduced. Penycuik is the Midlothian abode of Prof. Ewart, who has now for some years followed the breeding investigations that so long occupied Darwin; and though to the general public these are better known as the Zebra hybrid experiments, much valuable work has been done with Pigeons, Fowls, Dogs, and Rabbits. The result, as might be expected, leads to another nail in the coffin of our old fetish "species," and the dogma as to its immutability. "Among plants, hybrids are sometimes quite fertile; while some crosses are quite, or almost, sterile. There is no hard and fast line between species and varieties, and hence there can be no fundamental difference between a hybrid and a cross, nor yet any a priori reason why any given hybrid should be sterile, or any given cross fertile. It is no longer possible to contend that species were originally endowed with mutual sterility, by way of preventing the confusion that would result from free interbreeding."
Prof. Ewart recognizes three distinct types of Zebras:—Equus grevyi, E. zebra, and E. burchelli, which, ignoring the now generally considered extinct E. quagga, is in agreement with the views of Mr. Pocock (cf. Zool. 1897, p. 380). He has bred nine Zebra hybrids by crossing mares of various sizes (from 11 to 15 hands) and breeds with his Zebra stallion, and possesses also three hybrids out of Zebra mares, one sired by a donkey, the other two by Ponies. The importance of these experiments is clearly seen by the separate considerations and discussions on such interesting biological problems or suggestions as—Reversion, Prepotency and Inbreeding, Telegony, Saturation, and Sterility; while the conclusion is reached that "there is obviously no real difference between cross-fertilization and intercrossing. Whether we interbreed or intercross, engage in 'line'
