Finally may be mentioned the curious variations in screech-owls of the genus Scops. This owl has two strikingly different plumages—a mottled gray and a reddish brown, which, although very distinct when fully developed, yet “are entirely independent of age, season, or sex.” There is an eastern form, _Scops asio_, that extends west to the Rocky Mountains. There is a northwestern form, _S. kennicotti_, which in its red phase is quite different from _S. asio_, but in its gray plumage is very similar. The California form, _S. benderii_, is not known to have a red phase, and the gray phase is quite different from that of _S. asio_, but like the last form. The Colorado form, _S. maxwellæ_, has no red phase, “but on the contrary the whole plumage is very pale, almost as if bleached, the difference evident in the nestlings even.” The Texas form, _S. maselli_, has both phases, and is very similar to _S. asio_. The Florida form is smaller and colored like _S. asio_. The red phase is the frequent, if not the usual, one. The flammulated form, _S. fiammula_, is “a very _small species_, with much the general aspect of an ungrown _S. asio_.” This is the southwestern form, easily distinguished on account of its small size and color from the other forms. These examples might be greatly increased, but they will suffice, I think, to convince one of the difficulty of giving a sharp definition to “species.” The facts speak strongly in favor of the transmutation theory, and show us how a species may become separated under different conditions into a number of new forms, which would be counted as new different species, if the intermediate forms were exterminated. In discussing the nature of the changes that bring about variability, Darwin remarks: “From a remote period to the present day, under climates and circumstances as different as it is possible to conceive, organic beings of all kinds, when domesticated or cultivated, have varied. We see this with the many domestic races of quadrupeds and birds belonging to different orders, with goldfish and silkworms, with plants of many kinds, raised in various quarters of the world. In the deserts of northern Africa the date-palm has yielded thirty-eight varieties; in the fertile plains of India it is notorious how many varieties of rice and of a host of other plants exist; in a single Polynesian island, twenty-four varieties of the breadfruit, the same number of the banana, and twenty-two varieties of the arum, are cultivated by the natives. The mulberry tree of India and Europe has yielded many varieties serving as food for the silkworm; and in China sixty-three varieties of the bamboo are used for various domestic purposes. These facts, and innumerable others which could be added, indicate that a change of almost any kind in the conditions of life suffices to cause variability—different changes acting on different organisms.”