During embryological development the fertilized ovum--consisting of two fused fragments of this germinal substance--gives rise to a host of ordered and marshalled cells, which are divisible into two groups: the one forms the body with its muscles, bones, glands, digestive system, skin, sense-organs, nerve-centres, and so forth; the other forms a reserve store of germinal substance, from which are derived the ova and sperms. The former take no direct share in reproduction; they are off the line of continuous descent; they die without issue. But they protect and minister to the reproductive function of the second group--the potential ancestors of the races to follow. But all instinctive and intelligent behaviour is the outcome of the orderly working of the nervous system, is initiated through sensory stimulation, and is executed by the motor organs; and all the structural parts, through which such behaviour is possible, belong to the body--that which dies without issue. How, then, can instinct and intelligence be inherited? In a sense they are not inherited. The nervous system which is their organic basis begets no heirs. But it is begotten of germinal substance, which not only produced the body of which the nervous system is a part, but also handed on, with that body, samples of the same germinal substance capable of reproducing a similar body and a like nervous system. Herein lies the basis of heredity. The stress of the struggle for existence falls upon the body; and instinctive or intelligent behaviour is a means to its preservation in the struggle for existence. According as it survives or not, will the samples of germinal substance it contains fulfil their biological end or perish with it. Natural selection secures the survival of those animals which bear the seed from which their like will be developed.