12. THIS FORM OF ISOLATION--WHICH, IN REGARD TO INCIPIENT SPECIES, I HAVE CALLED PHYSIOLOGICAL SELECTION--MAY ACT EITHER ALONE OR IN CONJUNCTION WITH OTHER FORMS OF ISOLATION ON COMMON AREAS: IN THE FORMER CASE ITS AGENCY IS OF MOST IMPORTANCE AMONG PLANTS AND THE LOWER CLASSES OF ANIMALS; IN THE LATTER CASE ITS IMPORTANCE CONSISTS IN ITS GREATLY INTENSIFYING THE SEGREGATIVE POWER OF WHATEVER OTHER FORM OF ISOLATION IT MAY BE WITH WHICH IT IS ASSOCIATED. _APPENDICES_ APPENDIX A. MR. GULICK'S CRITICISM OF MR. WALLACE'S VIEWS ON PHYSIOLOGICAL SELECTION. I have received from Mr. Gulick the results of his consideration of Mr. Wallace's criticism. As these results closely resemble those which I have myself reached, and as they were independently worked out on the other side of the globe, I deem it desirable to publish them here for the sake of comparison. In his covering letter Mr. Gulick writes:-- Mr. Wallace has most certainly adopted the fundamental principles of our theory, and in an arbitrary way attempted to claim the results produced by these principles as the effects of natural selection. He takes our principles, which in the previous chapter he has combated; but he makes such disjointed use of them that I am not willing to recognize his statement as an intelligible exposition of our theory.... I have endeavoured to indicate at what points Mr. Wallace has deserted his own principles, and at what points he has failed to make the best use of ours. To bring out these points distinctly has been no easy task; but if you regard this paper on _The Preservation and Accumulation of Cross-infertility_ as giving any help in elucidating the true principles, and in showing Mr. Wallace's position in regard to them, I shall be satisfied. Please make any use of it that may seem desirable, and then forward it to Professor Dana. The following is a general summary of Mr. Gulick's results:--