What has been so far said about the germ plasm has been to enable the reader to possess a degree of intelligence on the nature of fertilization, so far as it is known; but from a practical standpoint the most important knowledge for those prospective parents who wish to practice intelligent stirpiculture is to understand that the health of the germ plasm or fertilized ovum depends on the health of the parents. By health, I mean the possession of a good constitution, to which will be added a strong hold on life, power to do and to endure, and quickly to recover from weariness. Disease will be easily warded off in such persons, so that there will be generally good health. Such a condition of body is usually inherited. It depends on the possession of a large supply in the body of living matter--firm muscles, a good heart, lungs and digestive organs. Those who are feeble cannot endure much; whose heart, lungs and digestive organs are weak; whose hold on life is slight, can rarely endow their offspring with these high qualities. Their children may live if no great strain comes upon them; but if they must take an active part in the struggle and competition going on in the world they cannot endure it. Mr. Spencer puts the case very aptly in his work on Ethics where he says: "It results that where maternal vigor is great, and the surplus vitality consequently large, a long series of children may be borne before any deterioration in their quality becomes marked; while, on the other hand, a mother with but a small surplus may soon cease altogether to reproduce. Further, it results that variations in the state of health of parents which involves variations in the surplus vitality have their effects on the constitutions of offspring to the extent that offspring borne during greatly deranged maternal health are decidedly feebler. And then, lastly and chiefly, it results that after the constitutional vigor has culminated, and there has commenced that gradual decline which in some twenty years or so brings absolute infertility, there goes on a gradual decrease in that surplus vitality on which the production of offspring depends, and a consequent deterioration in the quality of such offspring. This which is _a priori_ conclusion is verified _a posteriori_.