SELECTION A SECONDAKY AGENT. 369 



Mammals. All these examples consequently display, in a con- 

 spicuoits degree, those very phenomena on which Kramer calcu- 

 lates, but with the omission of an essential factor, namely, that 

 very factor which, according to Darwin, is, under the form of 

 ' sexual selection,' one of the chief agents in effecting the selection, 

 i.e. the preservation of extreme and particularly favoured varie- 

 ties. If, on the other hand, we take this factor into the calculation, 

 the numerous cases so strongly insisted on by Kramer, of great 

 constancy in the distinctive characters of male beetles, may be 

 very well explained by Darwin's principles. According to these 

 the selection effected in these instances by the physiological 

 bearings of the organs in question has already acted, and has 

 selected, and consequently rendered constant, those species 

 which had special advantages in the struggle which led to the 

 selection ; while in those cases of still prevailing variability for 

 which Kramer's calculation holds good, no such physiological 

 bearings can have been at work. For it must not be forgotten 

 that neither natural nor sexual selection can originate new 

 characters, but can only come into play when some active 

 mechanical causes have given rise to such modifications in 

 organs already existing, as are capable of introducing some new 

 physiological correlation. So long as this does not take place, 

 the forpe which originally gave rise to the deviations, i.e. the 

 variability, will still be able to act unchecked. Now the ques- 

 tion as to how an old organ may come to have a new physio- 

 logical value, in relation either to the other organs of the same 

 creature or to the external conditions of existence, is evidently 

 one of great importance ; and this seems to me a suitable oppor- 

 tunity for studying it with reference to an actual example, 

 namely, the eyes recently detected by myself' on the back of an 

 Onchidium, a naked mollusc. 



It is universally known that almost all univalve Mollusca 

 have two eyes either at the tip of the tentacles or at their base. 

 These eyes are extremely different as to structure from those of 

 the Yertebrata. In all eyes, without exception, the optic nerve 

 is gradually merged in a layer of tissue which includes the ter- 

 minating fibres of the nerve and is known as the retina ; theso 

 fibre ends are the rods and cones, or columnar layer. In Verte- 



