forms; and they must be higher, in so far as the later and more improved forms have conquered the older and less improved forms in the struggle for life; they have also generally had their organs more specialised for different functions. This fact is perfectly compatible with numerous beings still retaining simple and but little improved structures, fitted for simple conditions of life; it is likewise compatible with some forms having retrograded in organisation, by having become at each stage of descent better fitted for new and degraded habits of life.

Lastly, the wonderful law of the long endurance of allied forms on the same continent — of marsupials in Australia, of edentata in America, and other such cases — is intelligible, for within the same country the existing and the extinct will be closely allied by descent.