Sir Oliver Lodge to F. Bernard Vesper, Jr., 3 February 1925:

Biologically it is difficult or impossible to draw a hard and fast line between one species and another: I see no reason why such sharp lines should be attempted.

As to Survival, we must be guided by the facts; I expect that there are many grades of survival, that is to say I expect that life never goes out of existence. But whether individuality is associated with it depends upon the grade attained.