269 



5. Are there external sexual differences, and is the amount and extent of 

 variation different in the sexes ? 



This determination can usually be left till later; it is introduced here so as 

 not to mar the sequence of the following points. 



6. Is there a successive modification in going from younger to older speci- 

 mens indicating a structural modification with age ? 



It may be possible with some species, for instance, that the number of rays 

 increases directly with the age. Should such <i case exist it might give rise to 

 entirely erroneous notions as to the influence or effect of selective destruction. 



7. Is the variation of each year grouped about a mean common to all the 

 specimens, or is each year's variation grouped about a center of its own? 



While the idea of the annual variation or the reaction of each brood to a 

 slightly varying environment was supposed to be a possible element, and suggested 

 as such in my first announcement of the station, I was entirely unprepared for the 

 startling annual variation in such a prominent character as the number of dorsal 

 spines which has been discovered by Mr. Moenkhaus and reported upon in another 

 paper. 



The neglect of the consideration of the environment during the early period 

 of development in modifying successive broods in different ways may lead to en- 

 tirely erroneous ideas of the structural modifications of growth on the one hand, 

 or the entirely erroneous ideas of the action of selective destruction on the other. 

 To determine the latter it is absolutely necessary to take individuals of the same 

 year's broods at successive periods or successive years. Whether as great an annual 

 fluctuation is present in crabs as has been observed in Etheostoma I can not pre- 

 sume to say. But the entire neglect of this element vitiates the results of Prof. 

 Weldon, of the committee of the royal society for "Conducting statistical in- 

 quiries into the measurable characteristics of plants and animals," which Mr. 

 Thistelton-Dyer (Nature, Mch., 1895. considers to be "among the most remarkable 

 achievements in connection with the theory of evolution." 



I quote from Prof. Weldon to show his methods and results. (Nature, Mch. 

 7, 1895, p. 449.) "In order to estimate the effect of small variations upon the 

 chance of survival, in a given species, it is necessary to measure, first, the percent- 

 age of young animals exhibiting this variation ; secondly, the percentage of adults 

 in which it is present. If the percentage of adults exhibiting the variation is 

 less than the percentage of young, then a certain percentage of young animals has 

 either lost the character during growth, or has been destroyed. The law of growth 

 having been ascertained, the rate of destruction may be measured, and in this 

 way an estimate of the advantage or disadvantage of a variation may he obtained. 



