But apart from such more speculative considerations there are yet other aspects to be considered, such as the effect of the environment, the conditions of the home, and the type of union between the male and the female, all of which have their influence on the duration and kind of care shown by animal parents to their offspring. Some explanation must be sought for the almost bewildering diversity which we find in this relationship; for while the young of some animals (and often among low types where least we should expect it) are jealously guarded and cared for by at least one of the parents, in others there is no trace of such sacrifice and solicitude, and the young are thrown on to the world, orphaned before they are born, and left to live or die as chance decrees. Why is this? Why is the parental instinct so actively strong in some cases, so absent in others? Can we, indeed, hope to find the answer? If we can do this, we shall learn much to surprise and also to instruct us. In the higher types of animals, with the longer period of infancy, some amount of care for the young is always shown by the mother. All the mammals, without exception, nurse their offspring for a longer or shorter period. Among the birds the young in many species are tended by both parents, and we find many beautiful examples of parental fosterage and protection. But we find also species, like the cuckoos, which thrust the parental duties on to others; and there are others, such as the megapodes, where the mother trusts the incubation of her eggs to natural agencies, and after placing them in a position to get the heat generated by decaying vegetation or that derived from hot springs, leaves them, and exhibits no apparent care for the future welfare of the family.