Page:Popular Science Monthly Volume 64.djvu/93

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SHOBTEB ARTICLES AND DISCUSSION

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��principle, it is probable that his mis- interpretation arose from the asso- •ciation in his mind of my communica- tion with Mr. Cannon's paper on ' A Cytological Basis of Mendel's Law,' where unfortunately the error in ques- tion was not avoided. This paper I first saw after its publication.

In point of fact the cytological evi- dence on which Sutton based his sug- gestion 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 pa- ternal and maternal chromosomes may lie indifferently toward either pole, and that consequently all combinations of paternal and maternal chromo- somes may be produced in the gametes. To employ Sutton's graphic illustra- tion : 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

ABCD abCD aBcD

the groupings — ; — r- , - ^^;, - . — . "= ^ ^ abed ' A Bed' AbCd'

and so on, which gives a possibility of 16 difi"erent combinations in the gametes and of 256 in the zygotes or ofi'spring. If the number of chromo- somes 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, there- fore, in full harmony with the fact that offspring may show many dif- ferent combinations of characters indi- vidually traceable to four grandparents ■or a greater number of more remote ancestors.

Despite the immense range of mixed variation and inheritance thus per- mitted under the assumption, a point •of real difficulty, not touched on by

��Mr. Cook, is the relatively small num- ber of chromosomes as compared with that of transmissible characters; for if the chromosome-hypothesis, as de- veloped by Sutton, be valid, it would seem to follow that each chromosome stands not for one, but for many, char- acters, and these should form a co- herent 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 hy- pothesis may perhaps be obtained by the comparative study of variation in nearly related species that diff'er 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 prin- ciple, Sutton has endeavored to show that they do not invalidate the sug- gestions 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 explana- tion, 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 me- chanisms of cells has no ascertained basis of fact' [1. 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 ac- quaintance with the literature of em- bryology and cytology.

Edmund B. Wilson. Columbia University, September 24, 1903.

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