July 7, 1893.] 



SCIENCE. 



The prevalence of this disease at the present time will be seen 

 from the statistics taken from the health reports of the following 

 southern cities. The figures given represent the per cent of deaths 

 from consumption, as compared with the total mortality from all 

 causes ; and also the proportion in 1,000 per annum. 



It will be seen from the above table that the rate of mo-tality 

 in proportion to 1,000 of population per annum is nearly four 

 times as great among the colored people as among the white. It 

 is probable, however, that consumption is much less prevalent in 

 the country districts. 



I will now consider some of the causes that have probably pro- 

 duced this excessive death-rate from this disease. 



1. Unhealthy dwellings, often situated on narrow alleys, reek- 

 ing in filth and moral and physical pollution. 2. Improper food, 

 often of poor quality and lacking in quantity. 3. Insufficient 

 clothing and exposure in inclement weather. 4. Irregular habits 

 and a lack of a proper amount of sleep. 5. Excessive use of alco- 

 holic drink. 6. Ignorance concerning ihe laws of health. 7. Lack 

 of medical attention and good nursing. 



LETTERS TO THE EDITOR. 



,** Correspondents are requested to be as brief as possible. The ■ 

 is in all cases req^Ured as proof of good faith. 



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The editor will be glad to publish any queries consonant with the character 

 of the journal. 



Variation and Evolution. 



No branch of the study of natural history is more interesting, 

 or more likely to lead to valuable results, than that of the causes 

 of the large amount of variation which is exhibited by many 

 species of animals. 



If, as seems certain, what were at first varieties, in the process 

 of time, by increase of the differential characteristics, or simply 

 by these becoming permanent, originated new species, we are, 

 while studying the causes which favor these variations, at the 

 same time gaining an insight into those of the origin of species 

 themselves. 



No class of animals offers more favorable conditions for this 

 study than the terrestrial and fresh-water mollusca. The great 

 variety of conditions under which many species live, and the 

 numerous varieties into which they are divided, together with 

 the ease with which they may be collected and kept under obser- 

 vation, make them peculiarly suitable for our purpose. 



Darwin says in an extract from one of his letters which I have 

 lately seen : " In my opinion, the greatest error I have committed 

 has been in not allowing sufficient weight to the direct action of 

 environment, independently of natural selection.'' Probably 

 those changes which are commenced in a species by the influence 

 of environment are, in proce.=s of time, fixed by means of natural 



selection. That there is a preference exhibited for individuals of 

 a like variety, even where the variation cannot be supposed to 

 confer any benefit, may be proved by anyone who will observe 

 the pairing of that most variable species, both in color and band- 

 ing. Helix nemoralis, be will find, though with many exceptions, 

 that among the pairs which he may discover by the roadside, soon 

 after sunrise or in the evenings during spring and early summer, 

 that there is a decided preference shown by these animals for in- 

 dividuals similar to themselves, the red varieties prefer to mate 

 with those of their own color, as do the yellows: while, in a less- 

 degree, it will be foiind that the many-banded select mates among 

 their own class rather than from the one-banded or unicolorous 

 forms. 



That in the majority of instances, at least, the progeny in those 

 cases in which individuals of a similar variety have mated re- 

 semble the parents I have been enabled to prove by selective breed- 

 ing. I am still continuing these experiments, and hope to have 

 something further to say on the subject at a future time. Doubt- 

 less otheif species show preferences of this kind. I have referred, 

 however, to those of which I have most experience. Is it not 

 probable that Helix hortensis and H. nemoralis have been derived 

 from a common form in comparatively recent times through 

 varietal differences which have at last become specific? 



Malacologists in America have opportunities denied to us in the 

 old country. They have the great advantage of being able to study 

 the variations, in introduced species, which have been produced 

 as the consequences of that introduction. 



As species introduced into a new country, under different cli- 

 matic conditions to those under which they have previously lived, 

 are in some degree similarly circumstanced to species living through 

 climatic changes produced by alterations of land and water sur- 

 faces, etc., during the changes which all parts of the world have 

 undergone during the long geological ages, we have in their cases 

 a means of studying what changes certain conditions are able to 

 produce, and consequently of gaining an insight into the causes 

 which have helped to the development of our present fauna from 

 their remote ancestors of the past. We can study the effects of a 

 more equable climate in some parts, of greater heat or cold in 

 others, of more and less moisture, of changes in the food-plants, 

 of exposure to the attacks of new enemies, etc 



The more this subject is investigated, the more, I believe, "ill 

 become apparent the fact that all species possess latent powers 

 which the proper stimulus in the shape of alts-red circumstances, 

 such as those suggested, is capable of bringing into action for 

 the benefit of themselves and their descendants. 



The observations at present recorded relating to the causes of 

 variation are scattered through a large number of publication'^ 

 these, in a short series of articles for another journal, I have en- 

 deavored to bring together and arrange for reference. Some of 

 the causes which the various writers have assigned as probably 

 inducing variation may be mentioned. Deficiency of lime in the 

 soil produces thin, horny shells, and in some degree may cause 

 change in their shape. Moisture, when deficient, is supposed to 

 favor the formation of thick, white shells among the terrestrial 

 mollusca, while its extreme abundance prevents the formation of 

 colored bands in those species usually possessing them. Deficiency 

 of light (as in dense forests) has been referred to as the cause of 

 dull, unicolorous shells, while those mere exposed to its influence 

 are often gaily colored. Heat, combined with moisture, is con- 

 sidered conducive to l>rilliant coloring, with dryness as increasing 

 the influence of the latter, while among the fresh-water species 

 it tends to the production of fragile, dwarfed shells, overcrowding 

 among the latter having a nearly similar effect. Dense vegeta- 

 tion, impeding the progress of aquatic species, has been considered 

 a cause of scalariform varieties. Flowing and stagnant water are 

 well known to effect the Limnaeidm to a large extent. Muddy, 

 rocky, and sandy bottoms also have their effects. Food is im- 

 doubtedly an element of great importance in the manufac- 

 ture of varieties in its relative abundance and luxuriance, while 

 other circumstances have been observed where certain plants ex- 

 isted in unusual abundance. The presence of certain moUuscan 

 enemies has been found coincident with peculiar deformities, e.g., 

 that of Hydra viridis, with deformed examples of a species of 



