What reasons does Appin give for making cats, and tTobermory in particular, his subjects?

From the text:

“And do you really ask us to believe,” Sir Wilfrid was saying, “that you have discovered a means for instructing animals in the art of human speech, and that dear old Tobermory has proved your first successful pupil?”

“It is a problem at which I have worked for the last seventeen years,” said Mr. Appin, “but only during the last eight or nine months have I been rewarded with glimmerings of success. Of course I have experimented with thousands of animals, but latterly only with cats, those wonderful creatures which have assimilated themselves so marvelously with our civilization while retaining all their highly developed feral instincts. Here and there among cats one comes across an outstanding superior intellect, just as one does among the ruck of human beings, and when I made the acquaintance of Tobermory a week ago I saw at once that I was in contact with a “Beyond-cat” of extraordinary intelligence.