If all life forms have been produced by gradual expansion through selected mutations from a small beginning gene pool, organisms really should just grade into one another without distinct boundaries. Despite this, many species and even whole families remain inexplicably constant. The shark of today, for instance, is hardly distinguishable from the shark of 150 million years ago. According to Professor W.H. Thorpe, Director of the Sub-department of Animal Behavior at Cambridge and a world authority, this is the problem in evolution. According to the author's view, which I think nearly all biologists must share, the species is the only taxonomic category that has, at least in more favorable examples, a completely objective existence.
Higher categories are all more or less a matter of opinion.
And the earth brought forth grass, and herb yielding seed after his kind, and the tree yielding fruit, whose seed was in itself, after his kind. Species are groups of interbreeding natural populations that are reproductively isolated from other groups. There is no evidence of the origin of a hybrid between man and any other mammal. There is always a limit beyond which a species cannot be bred. After a few generations, they had reached a specific size and remained there, unable to vary further in either direction.