CHROMOSOME THEORY OF HEREDITY 241 



geographical varieties — i.e. incipient species — is afforded by 

 most of the recent work on variation in the field. Mention 

 need only be made of most of the instances quoted by Bateson in 

 chap, vii of his Problems of Genetics, and further of Crampton's 

 monumental work {Carnegie Publication, No. 228, 1916) upon the 

 snails of the genus Partula in Tahiti. The varieties of these 

 snails isolated in the various valleys were distinguished by differ- 

 ences which he found impossible to correlate with any difference 

 of environment, but which could be readily and simply conceived 

 as due to mutations in a smaller number of genes. Finally, 

 reference should be made to the important review by Sturte- 

 vant on the North American species of Drosophila {Carnegie 

 Publication No. 301, 1921), in which he shows that the charac- 

 ters separating species of Drosophila are often similar to the 

 mutations found in the laboratory. 



It should finally be remembered that this Mendelian 

 recombination has the consequence of producing, according to 

 the laws of probability, the extreme combinations rarely, but a 

 vast number of combinations which exhibit but few differences 

 from the average. 



We now come to the other aspect of the problem. Self- 

 regulating mechanisms are, one might say, the hall-mark of 

 life. Whether, as writers like Haldane appear to believe, they 

 are to be considered as the outcome of an inherent tendency 

 of living matter to organic regulation, or whether, as probably 

 the majority of biologists would prefer to hold, they are the 

 outcome of long selection upon living substance with its 

 fundamental power of self-reproduction, does not concern us 

 here. Enough to realise their importance. The constancy of 

 temperature in mammals and birds is usually quoted as the 

 typical example. But much more striking instances may be 

 adduced. The regulation of the acidity of the blood is so 

 accurate that an increase of one part of hydrogen ion in a 

 hundred million millions of blood (i : 100,000,000,000,000) is 

 responded to by a change in respiratory rate tending to restore 

 the normal concentration. The regulation of the salt-content 

 of the blood by the mammalian kidney is of the most consum- 

 mate accuracy. The regulation of organic form, as seen in 

 regeneration among the lower organisms, and in the early stages 

 even of the highest, is another example ; in normal environ- 

 ment, the organism restores just that part which is removed, 

 and the form-equilibrium is re-established. 



The chromosomes and their contained genes may be looked 

 upon as the regulating mechanism which makes heredity, 

 and indeed any real organic constancy from generation to 

 generation, possible at all — the regulating mechanism of 

 species. 



