852 



SCIENCE. 



[N. S. Vol. XXIV. No. 626. 



1. Homologous chromosomes must retain 

 sufficient similarity of function to function 

 together in synapsis and reduction. They 

 may vary in function, but must vary together, 



2. Non-homologous chromosomes must func- 

 tion together in mitosis and in synapsis and 

 reduction. Hence, if they vary in a given 

 interbreeding group, they must vary together 

 within narrow limits. 



3. Chromosomes must not disagree in the 

 expression of vital characters to such a degree 

 as to interfere with the development of a per- 

 fect organism. 



4. In non-essential characters that do not 

 interfere with proper functioning in mitosis, 

 synapsis, and reduction and in the develop- 

 ment of a perfect organism, chromosome func- 

 tions are free to vary. 



6. If a change in the expression of a char- 

 acter changes an essential relation between 

 the organism and its environment, natural 

 selection favors or retards variation according 

 as the variation affects the relation to environ- 

 ment favorably or unfavorably (protective 

 adaptation, for example). 



6. Chromosome variation is limited by the 

 constitution of the chromatin itself. 



Organisms that reproduce asexually are 

 freer to vary than others. An organism con- 

 tinuously propagated by cuttings may, by un- 

 restrained variation of chromosome function, 

 lose the power of sexual reproduction, e. g., 

 the banana plant. Presumably, variation is 

 here so rapid as to secure the advantages or- 

 dinarily conferred by sexual reproduction. 



Evolutionary changes may, in general, be 

 accounted for as the result of slow, gradual 

 changes in the functions of the chromosomes. 

 It was shown, however, that bud variations 

 are possibly marked changes in chromosome 

 function due to a new adjustment between 

 the constitution of chromatin and the food 

 supply or other elements of the environment. 

 Presumably, such changes are most likely to 

 occur in forms propagated asexually, and es- 

 pecially when propagated under conditions of 

 forced growth, where the food supply is ab- 

 normally large. Bud variations may possibly 

 be correlated with changes in the numbers of 

 chromosomes. 



It was pointed out that a widely distributed 

 species might present a series of forms, ad- 

 jacent sections of which might differ so little 

 as to interbreed freely, while the extremes, 

 if they met, might not be able to interbreed 

 at all. 



In the discussion which followed Dr. O. F. 

 Cook pointed out that the doctrine of the 

 permanent identity of the chromosome has 

 not been established, and that the indications 

 of the more recent cytological research were 

 against it. Mr. Spillman replied that a dif- 

 ferent interpretation might be placed upon 

 the cytological phenomena cited by Dr. Cook. 



In response to a question by Mr. M. B. 

 Waite, whether the view set forth by Mr. 

 Spillman, in addition to explaining geograph- 

 ical species, would not tend also to explain 

 what a species is, Mr. Spillman replied that 

 the theory formed important evidence on this 

 question, stating that while specific distinc- 

 tions differ in different groups and in the 

 minds of different investigators, we might go 

 so far as to say that two groups which can 

 not interbreed must be distinct species. Even 

 if they do not present morphological differ- 

 ences that will permit their identification 

 variation will soon bring about such differ- 

 ences in the absence of interbreeding. On 

 the other hand, the term species has acquired 

 such a meaning that we can not state that two 

 groups that can interbreeed are the same spe- 

 cies. Inability to interbreed may be taken 

 as the outer margin of the field of specific 

 distinction. 



Mr. Doolittle cited as examples analogous 

 to that of the goldenrod referred to by Mr. 

 Spillman (as a widely distributed group ad- 

 jacent sections of which interbreed but ex- 

 treme forms of which might not interbreed) 

 certain species of rodents described by Dr. 

 Merriam, and Dr. Stejneger mentioned two 

 butcher birds (Lanius), subspecies in Central 

 Asia which came together by different routes 

 in the Scandinavian peninsula as separate 

 species. M. 0. Marsh, 



Secretary. 



THE TORREY BOTANICAL CLUB. 



The meeting of the club on November 13, 

 1906, was called to order by President Eusby 



