THE FACTORS OF EVOLUTION 195 



illuminated or grow in the dark, I hope to have adduced a suffi- 

 cient number of facts to prove the formative power of the light. 



In botany an apparatus is frequently used in which a hori- 

 zontal axis is made to rotate slowly by clockwork. This clinostat, 

 as it is called, enables us to withdraw objects fastened to the 

 rotating drum from the influences of the law of gravitation. 

 The results obtained are in many cases very wonderful. For 

 instance, if we place climbing plants on the clinostat, the 

 turning movements of their stem cease, the younger growing 

 parts unravel, and the entire plant extends longitudinally. 



A still better instance of the influence of gravitation is given 

 in experiments by Massart, recently published by Lotsy. In the 

 manner mentioned Massart withdrew a tropical plant, Alloplectus 

 sanguinea, which is distinguished by its asymmetrical leaves, 

 from the influence of gravitation, without bringing about any 

 particular change ; even newly-formed leaves exhibited a similar 

 asymmetrical form. But if the seed was permitted to germinate 

 on the clinostat, Massart obtained plants with symmetrical 

 leaves. 



Interesting observations concerning the effects of altered 

 gravitation on the development of the eggs of a water-beetle, 

 Hydrophilus aterimus, were recently made by Megusar. Like 

 its better known relative the Great Water-beetle, H. piceus, this 

 insect builds peculiar boat-like cocoons which float on the surface 

 of the water. In the interior of the firmly closed fabric the eggs 

 stand in close vertical formation. The lower part of the eggs 



-k OO 



becomes afterwards the anterior side of the embryo. This 

 position remains unaltered during the normal course of develop- 

 ment, but when Megusar turned the whole cocoon upside down 

 and fixed it in this unusual position, thereby changing at the 

 same time the poles of the eggs themselves, he first of all retarded 

 their development, for the first larvae appeared several days later 

 than those which had come from a cocoon which had been left in 

 its normal condition. Furthermore, the size of these larvae was 

 much less than that of normal larvae ; they possessed plumper 

 bodies ; their movements in the water were awkward and clumsy ; 

 they endeavoured in vain to seize their prey, and after a few 

 days were found dead. 



There are in nature only very few cases in which the formative 



