January 25, 1907] 



SCIENCE 



153 



ments produced. The relation between agouti 

 and black is precisely the same as that be- 

 tween short hair and long hair due likewise to 

 diiferences in follicle activity, as I have else- 

 where shown, but inherited quite independently 

 of hair pigmentation. Short hair is the re- 

 sult of a determinate growth cycle; the hair 

 grows so long and then stops growing; long 

 or angora hair is the result of indeterminate 

 activity on the part of the hair follicle; the 

 hair keeps growing so long as its follicle is 

 alive. 



We are now able to give a rational explana- 

 tion of the origin of the various color varie- 

 ties of rodents. The wild cavy transmits in 

 all its gametes the three factors A, B and B. 

 By accident (mutation) a gamete has been 

 formed which lacked A. When two such 

 gametes came together the result is a black 

 individual, and this individual ivill breed 

 true. Here is the explanation of our occa- 

 sional black squirrels, porcupines and the like. 

 If by a further mutation B is lost, leaving R 

 alone, a red race is produced which will breed 

 true and will not give reversion on crossing 

 with hlachs. Such are ordinary red guinea- 

 pigs. 



But if mutation is directly from the wild 

 or agouti condition, ABB, by loss of B, leav- 

 ing AB, then there is produced a red not dif- 

 ferent from ordinary reds in appearance, but 

 which will give reversion in crosses with black. 



The albino mutation, which is frequently 

 found in wild as well as in tame rodents, is 

 not due, as might be supposed, to simultane- 

 ous loss of the three factors A, B and B, for 

 albinos can be shown to possess, some one, 

 some two and some all three of these factors. 

 They have, according to Cuenot, lost a cer- 

 tain other factor necessary for the production 

 of pigment of any kind, an activating or fer- 

 ment-like factor. 



It has been observed that one mutation is 

 often followed by another. De Vries in his 

 Mutaiion-stheorie speaks repeatedly of periods 

 of mutation. We can begin to see the signi- 

 ficance of this; given one mutation, we can 

 produce others. 



Suppose, for example, that we possess agouti 

 and ordinary red varieties only and desire 



black, we are not compelled to await a muta- 

 tion to produce it; we can cross red with 

 agouti and obtain black in the second genera- 

 tion. This is not hypothesis merely; its cor- 

 rectness has already been in part demonstrated. 

 Thus, in one experiment, there was employed 

 an agouti of the formula AB -AB, which gave 

 only reds and agoutis in crosses with red, but 

 the agoutis so produced when mated in the 

 same way as the parent gave blacks as well as 

 reds and agoutis, for they were of the formula 

 AB -R. From such animals homozygous 

 blacks (B-B) are readily obtained. 



To produce a red variety from agoutis and 

 blacks alone would not be so easy; it would 

 be necessary either to await a mutation or to 

 work by the slow process of selection from 

 continuous variations in the intensity of 

 blacks under cross-breeding with agoutis. In 

 mice and rabbits as well as in guinea-pigs red 

 (or yellow) varieties are well known, but in 

 rats yellow has never been obtained separate 

 from black, though black and agouti varie- 

 ties are conmion, both wild and in captivity. 



We now know what the ' fixation ' of a 

 heterozygous character implies. When A and 

 B are crossed, we obtain C is due either 

 simply to co-existence of A with B, or to the 

 co-existence with them of a third factor intro- 

 duced with one or the other. In either case 

 fixation will consist in getting into the gamete 

 all the factors which produce C. In the first 

 case, the zygote is A-B, and the resultant is 

 equivalent to C. Fixation will consist in 

 getting a zygote of the formula AB -AB; 

 every gamete produced will then bear the 

 equivalent of C, viz., AB. In the second case, 

 the zygote is either AC -B or A -OB; fixation 

 will consist in obtaining a zygote, ACB -ACB; 

 every gamete formed will then contain the 

 three factors, A, C and B. W. E. Castle 



Zoological Labobatory, 

 Harvard University, 

 December 26, 1906 



BOTANICAL NOTES 



THE RUSTS OP AUSTRALIA 



Under this title D. McAlpine, the govern- 

 ment pathologist of Victoria, Australia, pre- 

 pared a book of 350 pages, which has been 



