Page:Popular Science Monthly Volume 64.djvu/93

SHORTER ARTICLES AND DISCUSSION 89 principle, it is probable that his misinterpretation arose from the association in his mind of my communication with Mr. Cannon's paper on 'A Cytological Basis of Mendel's Law,' where unfortunately the error in question was not avoided. This paper I first saw after its publication.

In point of fact the cytological evidence on which Sutton based his suggestion leaves quite undecided the question whether any definite order is followed in the grouping of the chromosome-pairs in the equatorial plate, and places no obstacle in the way of assuming that their position is a matter of chance, i. e, that paternal and maternal chromosomes may lie indifferently toward either pole, and that consequently all combinations of paternal and maternal chromosomes may be produced in the gametes. To employ Sutton's graphic illustration: if the number of chromosomes be taken as 8 and designated as A, a, B, b, C, e, D, d (large letters denoting paternal chromosomes and small ones the corresponding maternal), the chromosome-pairs in the equatorial plate might, so far as the cytological evidence shows, present any or all the groupings $ABCD⁄abcd'$ $abCD⁄ABcd'$ $aBcD⁄AbCd'$ and so on, which gives a possibility of 16 different combinations in the gametes and of 256 in the zygotes or offspring. If the number of chromosomes be 24 (a very common number), the number of possible combinations in the gametes becomes more than 4,000 and in the zygotes nearly 17,000,000 (Sutton). The assumption is, therefore, in full harmony with the fact that offspring may show many different combinations of characters individually traceable to four grandparents or a greater number of more remote ancestors.

Despite the immense range of mixed variation and inheritance thus permitted under the assumption, a point of real difficulty, not touched on by Mr. Cook, is the relatively small number of chromosomes as compared with that of transmissible characters; for if the chromosome-hypothesis, as developed by Sutton, be valid, it would seem to follow that each chromosome stands not for one, but for many, characters, and these should form a coherent group in inheritance. Coherent groups of associated characters have, however, been recognized by many observers, including Mendel himself; and in this direction definite evidence for or against the chromosome hypothesis may perhaps be obtained by the comparative study of variation in nearly related species that differ in the number of chromosomes, though this presents a problem of great complexity. Regarding cases of non-conformity to the so-called Mendelian law or principle, Sutton has endeavored to show that they do not invalidate the suggestions given by the cytological work of himself, Montgomery, Cannon and others. They sufficiently indicate, however, that these suggestions do not yet afford a full or positive explanation, but only, in my own former phrase, give a 'clue' which awaits further development and test. It is entirely possible that the clue may prove false, yet even so it may serve to illustrate that 'fertility of false theories' to which Mr. Cook pays his tribute. In the meantime it is to be regretted that a biologist of Mr. Cook's standing should give currency to the statement that 'The notion that heredity, variation and evolution are the functions of special organs or mechanisms of cells has no ascertained basis of fact' (l. c., p. 222). This 'notion' may be true or false, but such an utterance will be truly surprising to any one having some degree of acquaintance with the literature of embryology and cytology.