CHAPTER XV. 

 VARIATION AND MONSTROSITIES. 



The study of variation is an important aid to phylogeny, for with the ever 

 shifting conditions within and around a center of life, that which is now an occa- 

 sional phenomenon, or "abnormality," may under other conditions become a 

 " normal" or constant result. Thus the abnormal of to-day may have been the 

 normal of yesterday, or the normal of to-day the abnormal of tomorrow. 



The minute variations expressed toward the close of development, and which 

 at most are only productive of new species, or even genera, are not likely to be 

 the sources of those fundamental changes which give rise to new classes. In our 

 problem, therefore, we should consider those early embryonic variations in verte- 

 brates that are likely to reveal the structure of their remote ancestors; or failing 

 that, we should seek for embryonic variations in arthropods that might have 

 produced a vertebrate; for if we know the broad limitations to the range of varia- 

 tion in a given animal, we may feel a reasonable confidence that we can roughly 

 define all the principal forms in which the ancestors or the descendants of such 

 animals could be expressed. 



To begin such a study, it is necessary to have an inexhaustible supply of em- 

 bryonic material that is easy to prepare, easy to observe, and easy to separate the 

 abnormal from the normal. I know no other animal in which these conditions are 

 so happily fulfilled as in Limulus. The eggs may be obtained in hundreds of thou- 

 sands and when properly prepared, the checkerboard arrangement of organs can be 

 easily observed in their normal and abnormal conditions. The abnormal embryos 

 may be obtained in great numbers by placing the eggs from many different nests in 

 running water. In due time, eight to ten weeks, the normal eggs hatch and the 

 free swimming larvae are carried off in the waste. The five or ten per cent, that 

 remain are apparently sound and healthy, but among them will be found all the 

 different phases of abnormality likely to occur. There are pygmies and giants, 

 in early and late stages; some are legless, others headless, or tailess, or consist 

 of fractional parts, such as halves, quarters and smaller divisions, in endless 

 combination and variety. Then there are doublets and triplets in various 

 stages of progressive or regressive growth. 



As all the eggs develop under similar conditions, the cause of these various 

 abnormal forms must be referred not to the unusual environment of a modern 

 hatching jar, but to variable conditions in the eggs themselves, that were probably 

 as frequent millions of years ago as they are to-day. 



These variations are much greater and more numerous than one might have 



274 



