_Phryniaxus nigricans_, at the breeding season, utters a call note which consists of two clear, musical “rings,” followed by a long descending “trill” like that of our British Greenfinch. But, it is to be noted, both sexes in this case perform. The period of sexual activity with perhaps the majority of animals is intermittent and extends over but a short period annually; with others potency is continuous, at least with the males, though desire becomes clamant only when aroused by external stimuli. But whenever this condition be aroused it invariably finds expression in exaggerated movements or vocal demonstrativeness. It uses the normal channels of expression, in short, but intensifies them. Now this period of sexual activity represents the maximum of “fitness” in animals, and it is not surprising, therefore, to find that when the barometer of vitality stands high some approach to the maximum of activity is indicated. In many birds this is revealed in song, though the earlier stimulating cause is absent. Among the cold-blooded frogs the same obtains. In the Edible Frog (_Rana esculenta_), for example, the males, which “are great musicians,” remarks Dr. Gadow, “go on singing for sheer enjoyment not only during the pairing time, but throughout the months of June and July. Warm, moonlight nights are the favourite times for the concert, which takes place in the water, beginning at sunset and continuing till early dawn. A few individuals utter a single note, ‘gwarr-oo-arr’ or ‘coarx’ but these are only preliminaries. The precentor ... begins with a sharp-sounding ‘brekeke’ and this is the signal for all the others to chime in with the same note, varied with all sorts of other sounds, bass, tenor and alto, each performer filling its resounding vocal sacs to bursting size, and these bags then look as if they acted as floats. When there are several hundred of these sociable creatures the din is continuous, and may be heard more than a mile off.” From what has been said of the Amphibia, and especially of the Newts, it would seem that, among the land vertebrates at any rate, the sexual instincts in this lowest or simplest form are satisfied with the discharge of the germinal products. Many, however, have advanced a stage further and reveal the rudiments of that instinctive care for offspring which develops to higher and higher grades as we ascend in the animal kingdom, till at last, in the human race, where the offspring is desired for its own sake, we ascend to the highest plane of all. The varied means of expression which these rudimentary instincts take in the Amphibia have already been discussed in “The Infancy of Animals,” which preceded this present volume, and hence no more need be said on this head in these pages. CHAPTER IX LOVE-MAKING AMONG FISHES