May 



1888.] 



SCIENCE. 



253 



the expression is sad and thoughtful, but by no means lacking in 

 intelligence. Of the patients suffering from paresis, one of the 

 women and three of the men had had apoplectiform seizures ; and 

 the average duration of the disease at the time of photographing, 

 was. in the women, two and one-third years, and in the men one 

 and three-fourths years. The average duration of paresis, before 

 it terminates fatally, is usually stated to be between three and four 

 years. Of the patients making up the composite of paresis, all, 

 with the exception of one woman, were in good general physical 



condition, and able to go out walking, and join in the usual round 

 of asylum-life : and this one woman was still able to go out walk- 

 ing on pleasant days, but was not so vigorous as the others. 



The composites seem fairly to represent the physiognomy of the 

 two diseases ; and that of paresis has been spoken of by several 

 alienists as being a typically characteristic face. The well-known 

 look of easy-going complacency of paresis is strongly shown in the 

 portrait. 



Mental diseases offer an excellent field for the study of types, 

 and it is to be hoped that further work in this line may give a more 

 just conception of the typical expression in the different forms of 

 mental disease than has hitherto been obtained from portraits of 

 individual cases. The portraits were first published in \}\t Journal 

 of Nervous and Mental Disease, and are reproduced here in the 

 hope that they will prove of interest to others than the medical 

 profession. William Noyes, M.D. 



New York, April 13. 



The Significance of 'Variety' and 'Species.' 



There is no question in biology more significant, or more diffi- 

 cult to answer, than what constitutes a species. Upon the answer 

 hinges the question of evolution, and more particularly the theories 

 of Darwin. In spite of an immense amount of discussion, no 

 answer has ever been given to the question which is in any degree 

 satisfactory. Certain it is that no definite amount of difference can 

 be regarded as enough or as too much to constitute a difference 

 between two species. The term 'species' is compatible with a 

 great amount of unlikeness on the part of varieties, or equally com- 

 patible with extremely small differences between species. Our 

 pigeons form an example of the first class ; and many species of 

 insects, of the second. In the former we find within the limits of a 

 single species an immense variety, the differences between the 

 varieties sometimes surpassing that between different families in a 

 state of nature. In the latter we have many species so closely like 

 each other as to require an expert to see any differences at all. It 

 is plain to every student that the term ' species ' is a variable one, 

 and its limits cannot be found in any definite amount of anatomical 

 variation. And yet, after all has been said concerning the indefi- 

 niteness of the term, every one will recognize that the word ' species ' 

 does mean something, and expresses some fact in nature ; that even 

 though, according to Darwinism, a species is merely an exaggerated 

 variety, yet there is a difference between a species with many 

 varieties and a genus with many species. The latter indicates, as. 

 every naturalist feels, a more fundamental difference of some sort, 

 even though to all appearances the differences may be less. Dar- 

 win did not regard the various pigeon^ as forming different species, 

 in spite of their unlikeness. 



This is not the place for a discussion of this matter, which 

 would involve the whole work of Darwin and his followers. There 

 is one suggestion, however, brought first prominently into notice by 

 Romanes {^Nature, August, 1886), which has not received the at- 

 tention it deserves, at least in this country. The suggestion is. 

 briefly this : that differences between species are due to the ac- 

 cumulation of differences in the sexual organs, entirely independ- 

 ent of anatomical differences. This idea does not belong exclu- 

 sively to Romanes, for it was independently suggested by at least 

 three others prior to the publication of the paper of Romanes 

 (CatCHPOLE, Nature, xxxi. p. 4 ; Belt, Naturalist in Nicaragua ; 

 and mj-self. Evolution of To-day, p. 41). Romanes alone, however, 

 expanded the view, and took upon himself to defend it against the 

 criticisms which were abundantly offered. In so doing he referred 

 to the principle of natural selection in such a way as to rouse the 

 enmity of many who revered Darwin's name and work, by claim- 

 ing that Darwin did not explain the origin of species at all, but 

 only the origin of adaptation. In thus seemingly attempting to- 

 belittle Darwin's discovery and relegate it to a very subordinate 

 position, Romanes called upon himself a severe criticism from 

 many who refused to see in his 'Physiological Selection ' any thing 

 new or important. These criticisms, though certainly showing that 

 Romanes had overrated the value of his principle in removing the- 

 difficulties in the way of the production of new species, did not by 

 any means show that this principle was not an important factor. 

 The idea is certainly new to literature ; and, although it may have 

 been hinted at by others, no one before Romanes formulated it so- 

 as to draw a clear distinction between anatomical and sexual vari- 

 ations. Whether or not the idea be regarded simply as a particu- 

 lar application of the principle of natural selection, as some of the 

 critics claim, is entirely immaterial to the value of the conception. 

 There is nothing in Darwin's writings to indicate that he had en- 

 tertained the thought that species are due to the selection of sexual 

 variations, while varieties are due to the selection of differences not 

 necessarily sexual. This idea, whether we regard it as an instance 

 of natural selection or not, certainly deserves careful study as- 

 promising to help in the solution of the puzzling problem of 

 species. 



There is no fact which has given rise to more discussion, or has 

 seemed to offer such difficulties in the way of Darwin, as the 

 alleged sterility of species when crossed. Many were the experi- 

 ments, and vast the amount of evidence collected, by Darwin for 

 the purpose of showing that the sterility of hybrids is not a law ; 

 and he did conclusively show that there is no absolute bar thus. 



