If you arrange nuts, or any other objects for that matter, in a curve according to size, you will find that the most numerous of them are of about the average size. This is equally true when applied to mankind. What is the reason?
There are a number of factors affecting this, but, in general, there are two main causes—environment and heredity. We do not know which is the more important but both are absolutely necessary.
In the picture being shown we see the influence of the black walnut upon plants around it. It creates an environment which influences the ability of other plants to grow near the roots.
It must be remembered, however, that what the animate plant transmits is not the actual character in question, but the ability of the animate plant to develop characteristics. By placing the plant near a black walnut tree we do not affect anything but the capacity of the plant to develop in certain directions.
I have shown here a diagram to illustrate a certain stock fertilization. Here we have the plant with its stamen and pistils, the egg cells and the pollen. There are two types of pollenization, one where the pistil is fertilized by insects carrying sticky pollen; the other by movement of the wind carrying the pollen. If I should believe my records, in attempts to cross trees, I might have a cross between a birch and an alder, in which the pollen is carried by the wind. I tried once to hybridize pines. I put some pitch pine pollen on the female flower of another species and seed resulted. I did this the second year and again I got seed. The third year I put bags on the female flowers before I could see them developing. Then I got no seeds. I believe that the pollen which had caused the seed to set in the preceding instances had come from the south for perhaps hundreds of miles.
There are times when the pollen of the staminate plant is all shed before the pistillate gets ready. Sometimes we have a plant that is self sterile. I have experimented with pollen from several different nut trees and also with the Norway spruce. Then again, there are abnormal cases; sometimes there is parthenogenesis. The jimson weed is the first plant which has ever been reproduced by parthenogenesis. Since that was discovered, an investigator in California has found a similar case in fruit developed without pollination.
One of the most important conceptions in heredity is its effect upon characters and factors. Take the Japanese bean here shown for example, one dark bean and one mottled. In the next hybrid generation we find three mottled and one dark. That is the familiar "three to one" ratio of Mendel's law. We believe now, that all, or at least a very large proportion of the heredity characters in plants of all kinds may be due to a series of factors; but the habit of growth of the plant is due to a single factor. We have the case here of a second generation of the weeping mulberry that I crossed with the white mulberry. As a result there was an average of three erects to one weeping one. Certain characteristics may be made up of the inter-action of a large number of factors. This will give a little idea as to the complexity of Mendel's law.
How do we get new characters in nature? New types are due to the rearrangement of previously existing characters, just as with the old-fashioned kaleidoscope, where you turn the crank and get new pictures. Another way is by the sudden appearance of new factors.
I wish to speak about one effect of hybridization, which is really connected with heredity factors, the vigor which occurs when we cross different varieties, species, or even races. In my experience certain types that have been naturally contrasted finally lose vigor, and after two or three generations the plant disappears. Then again I could show you cases where yields are greatly increased due to hybridity. These are established facts, not only as regards species of plants and trees but also as regards the human race. Hemy, in Dublin, who has done the best work in this line of endeavor, believes that many of our more rapid-growing trees are rapid-growing because they are hybrids.
To summarize, I have tried to point out the fact that diversity which we see in nature is real, and that it is brought about by two causes, namely, environment, and heredity. And that heredity is brought about by factors in the bodies of the chromosomes which are shuffled around like cards in a pack; they reappear in the same way that the cards will reappear. We have no means, as yet, of controlling the appearance of the factors, but we have two methods of getting new factors, as follows: