The history of these Bogota geese is most instructive. At the outset we meet with all those circumstances which would seem to justify us in the prediction of a failure. The infertility of the females, as attested by the rarity of the eggs, and that of the males, as shown in the strong proportion of addle eggs, point to a serious physiological injury to the organs whose action alone insures the permanence of the species. The enormous mortality among the young birds betrayed a no less serious alteration in the components of individual life. Nevertheless, at the time of M. Roulin’s journey, acclimatisation had been almost realised, and must without doubt now be completed. More than twenty years were, however, necessary for the organisation of this European bird to establish a harmony between itself and the conditions of existence on the high plateaus of America. The breeders were consequently forced to submit to many losses, affecting both generations and individuals. We see what took place in the case of the fowls and geese as well as in that of the wheat. Shortly after their emigration the climate killed all those who were unable to conform to the new exigencies. A certain number offered sufficient resistance to live almost as long as they would have done under their natural conditions of existence; but their weakened organisation was unfitted for generation, or could only produce beings which at once succumbed. Through all these disasters, however, a few privileged organisations conformed, from the first, more or less to the new exigencies. With slight modifications they transmitted their own acquirements combined with the suitable aptitudes to their progeny, who in turn made further advances in the direction opened by their parents; and from year to year the adaptation was more complete, the acclimatisation more nearly realised. But it is evident that years here represent generations. It is only from parent to offspring, through heredity and accumulation, that the living being becomes modified, and by degrees harmonises with the conditions of life. When, however, we are no longer studying an animal, plant, or a bird, which has the faculty of yearly reproduction, but species or races of a more tardy reproduction, we must remember that it is necessary to reckon by generations, and not by years.