Chapter V: , this hypothesis carries the assumption that every visible

character is due to the effects of not one but many factors in the germ-cell.

In addition to these fundamentals, there are numerous extensions and corollaries, some of them of a highly speculative nature. The reader who is interested in pursuing the subject farther must turn to one of the text-books on Mendelism.

In plant-breeding a good deal of progress has been made in the exact study of Mendelian heredity; in animal breeding, somewhat less; in human heredity, very little. The reason is obvious: that experiments can not be made in man, and students must depend on the results of such matings as they can find; that only a very few offspring result from each mating; and that generations are so long that no one observer can have more than a few under his eyes. These difficulties make Mendelian research in man a very slow and uncertain matter.

Altogether, it is probable that something like a hundred characters in man have been pointed out as inherited in Mendelian fashion. A large part of these are pathological conditions or rare abnormalities.

But the present writers can not accept most of these cases. It has been pointed out in Chapter V that there are good reasons for doubting that feeble-mindedness is inherited in a simple Mendelian fashion, although it is widely accepted as such. We can not help feeling that in most cases heredity in man is being made to appear much simpler than it really is; and that particularly in mental characters, analysis of traits has by no means reached the bottom.

If we were asked to make out a list of characters, as to the Mendelian inheritance of which there could be little doubt, we would hardly be able to go farther than the following:

The sex-linked characters (one kind of color-blindness, hemophilia, one kind of night-blindness, atrophy of the optic nerve, and a few other rare abnormalities).

Albinism. This appears to be a recessive, but probably involves multiple allelomorphs in man, as in other animals.

Brachydactyly, apparently a dominant. This is so much cited in text-books on Mendelism that the student might think it is a common character. As a fact, it is extremely rare, being found in only a few families. The similar trait of orthodactyly or symphalangism, which likewise appears to be a good Mendelian dominant, seems to exist in only one family. Traits like these, which are easily defined and occur very rarely, make up a large part of the cases of probably Mendelian heredity. They are little more than curiosities, their rarity and abnormal nature depriving them of evolutionary significance other than to demonstrate that Mendelian heredity does operate in man.

White blaze in the hair or, as it might better be called to show its resemblance to the trait found in other mammals, piebaldism. A rather rare dominant.[204]

Huntington's Chorea, which usually appears to be a good dominant, although the last investigators (Muncey and Davenport) found some unconformable cases.

A few abnormalities, such as a premature graying of the hair (one family cited by K. Pearson) are well enough attested to be admitted. Many others, such as baldness, are probably Mendelian but not yet sufficiently supported by evidence.

None of these characters, it will be observed, is of much significance eugenically. If the exact manner of inheritance of some of the more important mental and physical traits were known, it would be of value. But it is not a prerequisite for eugenic action. Enough is known for a working program.

To sum up: the features in the modern view of heredity, which the reader must keep in mind, are the following:

1. That the various characters which make up the physical constitution of any individual plant or animal are due to the action (concurrently with the environment, of course) of what are called, for convenience, factors, separable hypothetical units in the germ-plasm, capable of independent transmission.

2. That each visible character is due to the coöperative action of an indefinitely large number of factors; conversely, that each of these factors affects an indefinitely large number of characters.