There is no such thing, therefore, as infancy or parental care in the lowest orders of animal life; of which, one result is a gigantic mortality among their offspring. Enormous numbers of eggs are laid to ensure the preservation of the species when left to fend for themselves. The turbot, for instance, must deposit millions of minute glassy ova or the species would become extinct. Even among frogs the destruction of tadpoles is so great that provision must be made to allow for this loss. The fostering care of birds for their young at once permits a great reduction in the number of the offspring; but, though birds give evidence of some capacity for parental care, infancy, as such, is really confined to mammalian young. Even here it is curtailed in a vast number of species; but wherever it exists it stands for power to progress, and represents capacity for benefiting by, indeed depending upon, education, if only in the simple form of learning by imitation--a form familiar to readers of such books as Long's "Schools of the Woods." Plasticity is the hall-mark of progress; educability indicates a brain more or less competent to assimilate, to remember, to compare, to discriminate. This door of progress has been merely set ajar for even the higher apes; it is open to man only. The period of plasticity is evidently prolonged in proportion to the degree in which conscious intelligence has superseded mere brute force in promoting successful survival--that is to say, the transmission of _mental ability_ rather than of _physical strength_ postpones maturity. Man alone possesses in full the powers of selection and adaptation, of reason and of emotion, of memory and of mental originality, which are included in his rich heritage of life. If he is to realise his full potentialities, he must have protection for years after birth and an extended time for development. The immature infant must be fed, sheltered, and stimulated, if the inherent powers of adjustment to surroundings are to develop normally. But so great is the instability associated with human immaturity and future potentiality, that arrested development is too often the heavy penalty paid by the child for the ignorance and carelessness of his parents.[107] Faults of food and clothing, insufficient warmth, cleanliness, or exercise, premature work or precocious responsibility and independence, prolonged overstrain or insufficient stimulation of mind and body, are the prevalent causes by which a child's normal growth is warped and prejudiced. Where this occurs he never enters into his birthright of power; it has too often been thoughtlessly bartered by his natural guardians, literally for a mere mess of pottage. X. CAUSES WHICH MENACE HEALTHFUL INFANCY AND CHILDHOOD