In our own experiments it can be shown that the circulation in the embryo was deficient in all cases where the eyes degenerated. The hybrids between _Fundulus_ and _Menidia_ have often a beating heart but rarely a circulation (although they form blood); and the same phenomenon occurred in the embryos which were exposed to a low temperature at an early period of their lives. Hence all the facts agree that conditions which lead to an abnormal circulation (and consequently also to an abnormal or inadequate nutrition of the embryonic eye) may prevent development and lead to the formation of blind fishes. Eigenmann states that no blood-vessels enter the eye of the blind cave salamander _Typhlotriton_. The presence or absence of light does not usually interfere with the circulation or nutrition of the embryonic eye, and hence does not as a rule lead to the formation of degenerated eyes. This would lead us to the assumption that the blind fish owe their deficiency not to lack of light but to a condition which interferes with the circulation in the embryonic eye. Such a condition might be brought about by an anomaly in the germ plasm or in one chromosome, the nature and cause of which we are not able to determine at present; but which, since it occurs in the germ plasm or the chromosomes, must be hereditary. This would explain why it is, that animals with perfect eyes may occur in caves and why perfectly blind animals may occur in the open. It leaves, however, one point unexplained; namely, the greater frequency of blind species in caves or in the dark and the relative scarcity of such forms in the open. Eigenmann has shown that all those forms which live in caves were adapted to life in the dark before they entered the cave.[277] These animals are all negatively heliotropic and positively stereotropic, and with these tropisms they would be forced to enter a cave whenever they are put at the entrance. Even those among the Amblyopsidæ which live in the open have the tropisms of the cave dweller. This eliminates the idea that the cave adapted the animals for the life in the dark. [277] Cuénot has proposed the term preadaptation for such cases and this term expresses the situation correctly. Cuénot, L., _La Génèse des Espèces animales_. Paris, 1911. Only those animals can thrive in caves which for their feeding and mating do not depend upon visual mechanisms; and conversely, animals which are not provided with visual mechanisms can hold their own in the open, where they meet the competition of animals which can see, only under exceptional conditions. This seems to account for the fact that in caves blind species are comparatively more prevalent than in the open.