We have seen that the nervous system not only regulates most of the existing functions of the body, but that it has indirectly influenced the development of various bodily structures and certain mental qualities, and that these powers of mind depend on the development of the brain.
By our guides in this matter, we are assured that the most important difference observed between man and the lower animals is the conscience; hence, if we would understand how it has been possible for man to rise to his present position, we must know something of the processes involved in the development of the social instincts, through which have originated conscience and a desire for the welfare of others outside of self. The importance of these instincts in the development of conscience is thus set forth by Mr. Darwin:
Any animal whatever, endowed with well-marked social instincts, the parental and filial affections being here included, would inevitably acquire a moral sense or conscience, as soon as its intellectual faculties had become as well, or nearly as well, developed as in man.
Sympathy, we are told, is the foundation-stone of the social instincts. From facts which are everywhere presented among the forms of life below man, it is evident that sympathy was developed at an early stage of animal life. It is doubtless strongly manifested in our ape-like progenitors, and it was probably this instinct which subsequently led to a community of interest and the coherence of the tribe.
In a consideration, therefore, of this question of sex development and the origin of the progressive principle, if, as we are assured, sympathy constitutes the foundation-stone of the social instincts, and if it is to these instincts that we are to look for the origin of the moral sense, or conscience— a faculty which constitutes the fundamental difference between the human species and the lower orders of life—the question naturally arises: In which of the two diverging lines of sexual demarcation has arisen sympathy, or an interest in the well-being of others? For an answer to our question we must look carefully to the facts connected with the development of the sexes within one of which have been acquired characters tending toward the welfare of society, or of individuals outside of self; within the other, characters looking only toward selfish gratification. Within the former, the maternal instinct predominates; within the latter, passion.
Mr. Darwin admits that “parental and filial affection lies at the base of the social instincts,” and gives as his opinion that this quality is the result of Natural Selection—that those individuals which bestowed upon their offspring the greatest care and attention, would survive and multiply at the expense of others in which this instinct was less developed. Therefore, in pursuing the inquiry of sex-function and sex-development, a question of considerable significance is at this point suggested: Within which parent is observed the greater tendency to bestow care and attention upon offspring?
We are assured that “the animal family is especially maternal.” So soon as a female bird has laid her eggs, she is animated only by one desire; neither the promise of abundant food nor the fear of bullets is able to divert her purpose. Although the males among the more highly developed birds assist in rearing the family, amongst various species it is only the female which cares for the young. The male duck has no interest in his progeny, neither has the male eider. Of the male turkeys Mr. Letourneau says that they
do much worse: they often devour the eggs of their females, and thus oblige the latter to hide them. Female turkeys join each other with their young ones for greater security, and thus form troops of from sixty to eighty individuals, led by the mothers, and carefully avoiding the old males, who rush on the young ones and kill them by violent blows on the head with their beaks.[44]
The males of various other species, jealous of the attentions of the mothers during the time that their efforts are directed toward the maintenance of their brood, often kill their young. Regarding the subject of paternal care, Mr. Letourneau observes: “It is important to notice that amongst birds, the fathers devoid of affection generally belong to the less intelligent, and are most often polygamous.”
By observing the habits of cuckoos the fact has been ascertained that among them the maternal instinct is almost entirely lacking. Of the cuckoo it has been remarked that it is a “discontented, ill-conditioned, passionate, in short, decidedly unamiable bird.” Its note is typical of its habits and character.