320 GRACE MEDES 



parts according to the law of frequency error, in the same man- 

 ner as Galton found to be the case for man . . . ." He 

 arrived at the conclusion "that the larvae show a considerable 

 variation in size apart from any influence caused by conditions 

 of environment. These variations Weismann considers to be 

 due to actual differences of the germ plasm." 



Moreover, he demonstrated that changes in the surrounding 

 medium increase considerably the extent of variability, and 

 so, he argued, offer a wider opportunity for the operation of 

 natural selection. Having determined as accurately as possible 

 the normal size and variability, he investigated the modifica- 

 tions consequent upon seasonal changes and such alterations 

 of the medium as might occur in nature; as, for instance, changes 

 of concentration of the sea-water, changes of temperature, 

 changes of amount of oxygen and of carbon dioxide, and changes 

 produced by addition of uric acid and of urea. As Vernon's 

 object was primarily to determine the extent of ultimate vari- 

 ation produced by the different experimental conditions to which 

 he subjected the ova and embryos, his measurements were taken 

 at the eighth day of development, the time when the larva of 

 Strongylocentrotus lividus normally attains its maximum size. 

 This offered no opportunity for the observation of any gradual 

 adjustment of the organism to its changed environment or any 

 progressive lessening of its power of resistance, as could be 

 detected by measurements at frequent intervals. 



One of the earliest investigations of the effects of changes 

 of the constitution of sea-water upon normal development of 

 the sea-urchin was made by Pouchet and Cambry ('89). Since 

 calcium is the element most concerned in formation of the skele- 

 ton, they concluded that decrease in the relative amount of this 

 constituent of sea-water must produce some characteristic effect 

 upon growth; that this effect would not be incurred by the 

 skeleton alone, because it is an integral part of the tissues of the 

 animal ; and hence that modifications of those tissues from which 

 it is built must also be involved. They failed in an attempt to 

 raise larvae in an artificial Ca-free medium, but succeeded in 

 doing so in sea-water in which as much as nine-tenths of the 



