Among the lower Vertebrata it rarely happens that both parents jointly take care of their progeny. M. Milne Edwards states, indeed, that in the Pipa, or Toad of Surinam, the male helps the female to disburthen herself of her eggs;[17] and the Chelonia are known to live in pairs. “La femelle,” says M. Espinas, “vient sur les plages sablonneuses au moment de la ponte, accompagnée du mâle, et construit un nid en forme de four où la chaleur du soleil fait éclore les œufs.”[18] But it may be regarded as an almost universal rule that the relations of the sexes are utterly fickle. The male and female come together in the paring time; but having satisfied their sexual instincts they part again, and have nothing more to do with one another. The Chelonia form, with regard to their domestic habits, a transition to the Birds, as they do also from a zoological and, particularly, from an embryological point of view. In the latter class, parental affection has reached a very high degree of development, not only on the mother’s side, but also on the father’s. Male and female help each other to build the nest, the former generally bringing the materials, the latter doing the work. In fulfilling the numberless duties of the breeding season, both birds take a share. Incubation rests principally with the mother, but the father, as a rule, helps his companion, taking her place when she wants to leave the nest for a moment, or providing her with food and protecting her from every danger. Finally, when the duties of the breeding season are over, and the result desired is obtained, a period with new duties commences. During the first few days after hatching, most birds rarely leave their young for long, and then only to procure food for themselves and their family. In cases of great danger, both parents bravely defend their offspring. As soon as the first period of helplessness is over, and the young have grown somewhat, they are carefully taught to shift for themselves; and it is only when they are perfectly capable of so doing that they leave the nest and the parents. There are, indeed, a few birds that from the first day of their ultra-oval existence lack all parental care; and in some species, as the ducks, it frequently happens that the male leaves family duties wholly to the female. But, as a general rule, both share prosperity and adversity. The hatching of the eggs and the chief part of the rearing duties belong to the mother,[19] whilst the father acts as protector, and provides food, &c.