the chemical constitution of their fluids and the nature of their tissues are often modified. (12/8.  Numerous cases together with references are given in my ’Variation under Domestication’ chapter 23 2nd edition volume 2 page 264.  With respect to animals, Mr. Brackenridge ‘A Contribution to the Theory of Diathesis’ Edinburgh 1869, has well shown that the different organs of animals are excited into different degrees of activity by differences of temperature and food, and become to a certain extent adapted to them.) Many other such facts could be adduced.  In short, every alteration in the function of a part is probably connected with some corresponding, though often quite imperceptible change in structure or composition.

Whatever affects an organism in any way, likewise tends to act on its sexual elements.  We see this in the inheritance of newly acquired modifications, such as those from the increased use or disuse of a part, and even from mutilations if followed by disease. (12/9.  ’Variation under Domestication’ chapter 12 2nd edition volume 1 page 466.) We have abundant evidence how susceptible the reproductive system is to changed conditions, in the many instances of animals rendered sterile by confinement; so that they will not unite, or if they unite do not produce offspring, though the confinement may be far from close; and of plants rendered sterile by cultivation.  But hardly any cases afford more striking evidence how powerfully a change in the conditions of life acts on the sexual elements, than those already given, of plants which are completely self-sterile in one country, and when brought to another, yield, even in the first generation, a fair supply of self-fertilised seeds.

But it may be said, granting that changed conditions act on the sexual elements, how can two or more plants growing close together, either in their native country or in a garden, be differently acted on, inasmuch as they appear to be exposed to exactly the same conditions?  Although this question has been already considered, it deserves further consideration under several points of view.  In my experiments with Digitalis purpurea, some flowers on a wild plant were self-fertilised, and others were crossed with pollen from another plant growing within two or three feet’s distance.  The crossed and self-fertilised plants raised from the seeds thus obtained, produced flower-stems in number as 100 to 47, and in average height as 100 to 70.  Therefore the cross between these two plants was highly beneficial; but how could their sexual elements have been differentiated by exposure to different conditions?  If the progenitors of the two plants had lived on the same spot during the last score of generations, and had never been crossed with any plant beyond the distance of a few feet, in all probability their offspring would have been reduced to the same state as some of the plants in my experiments,—­such as the intercrossed plants of the ninth generation of Ipomoea,—­or