REPORT OF THE DIRECTOR I919 65 



spines (over 3.3 mm long) correspond to the seventh to ninth pair of 

 thecae, in the specimens observed. 



We have thus four mutations of Gl, ossograptus 

 quadrimucronatus which appear at about the same time 

 and which are all distinguishable by one or several rings of greatly 

 lengthened spines, some distance from the sicular end. When less 

 closely observed, all four would be readily taken for one or the same 

 mutation. The Holland Patent mutation bears, however, the longer 

 spines at the fifth to seventh thecae, the Herkimer county form at 

 the seventh to ninth pair, the British form at the tenth pair and the 

 Australian form at the seventh or eighth pair. 



It is not possible that the Herkimer county form is identical with 

 either the British or the Australian one, for it still differs from both 

 though less widely than the Holland Patent form. 



There are thus four different mutations of the species which have 

 developed at practically the same time in the same direction, namely, 

 the enlargement of a ring of spines. In Great Britain there occurs, 

 further, a very closely related species, Orthograptus 

 pageanus var. abnormispinosus Elles ■& Wood 

 which, at the same time, also bears a ring of four longer spines, at a 

 similar distance from the sicular end. 



There existed thus some stress or tendency which led independently 

 in widely separated regions to the development of similar or hom- 

 oeomorphic mutations which, however, are not identical, as proved 

 by the slightly different location of the rings of spines. If by acci- 

 dent, the rings had appeared at the same level of the rhabdosome, 

 those mutations would be indentical although they had originated 

 independently and separately in different regions, as proved by the 

 fact that they do not occur together in any of the regions, which, in 

 that case would constitute the center of distribution. One naturally 

 asks. Have such cases not happened oftener and the forms been 

 thrown together into one species of wide distribution, because the 

 original differences, denoting independent evolution, have failed of 

 observation ? It would then seem possible that even the same species 

 (or what to our crude discernment appears as one species, but really 

 is not), may at times have originated in several separate regions inde- 

 pendently, a possibility that has been repeatedly denied by students 

 of the principles of evolution. It would be only necessary, it seems 

 from the case here described, that a species of worldwide distribu- 

 tion, be exposed to the same influence or stress at separate places 

 and then respond in exactly the same manner. It could also be 

 urged that there might have existed a tendency to a development of 



