m'clung: cytology and taxonomy. 215 



parts is sufficiently close to allow the use of purely external 

 structures, but in other instances the entire anatomy will have 

 to be understood. It has been a matter of great interest to me 

 to find how real are the distinctions between species that have 

 been established by systematists. In the genus Hesperotettix, 

 that I have been using as an example, the species are clearly 

 marked by peculiarities of external structure, color, etc., and 

 these are strictly correlated with germ-cell characters. 



These things all speak strongly to me of definiteness and 

 thoroughness of organization, a conception that we ordinarily 

 express as "correlation of characters." It means, in truth, that 

 an organism is a very complex assemblage of parts, each of 

 which at any particular time has a definite form and relation to 

 the other parts. To know it thoroughly we must be familiar 

 with all its parts at that particular stage which it has reached 

 in its development. An organism is an organism, no matter at 

 what ontogenetic period it is regarded. Practically all crus- 

 taceans, as we know, at one stage of their existence have a form 

 called a "nauplius." The definitive organization may express 

 itself by only slight modifications of the early type of structure, 

 or it may progressively alter its form until it bears no resem- 

 blance to the former condition. But these nauplii are not all 

 alike, and doubtless every form has a different expression for 

 this structural type could we but distinguish it. Not only is 

 this true, but in all probability every cell is characteristically 

 different. 



With this conception of organisms we see that in their study 

 the distinction between cytology and taxonomy is not great. 

 Were our knowledge of cell structure in the grasshopper com- 

 plete enough we might erect a system of classification based 

 upon cytological characters, just as reasonably as we have des- 

 ignated one using external anatomical structures. All of which 

 goes to show that organisms at all stages of development and in 

 all their parts are specifically constituted. It is the peculiar 

 privilege of cytology not only to recognize these differences 

 but to determine the means by which they came about. The 

 apprehension of large principles of organization should there- 

 fore be our aim, and I have no doubt that once an understand- 

 ing of the cytological changes in the body of an animal during 

 its ontogeny is reached we shall have solved, as far as it is 

 possible for us to do, some of the larger problems of heredity 

 and development that have become our scientific inheritance. 



