THE NATURALIST S OCCUPATION. 5 I 



My anticipations seem to have been in the right direc- 

 tion, so far as I can judge from the observations of 

 Dr. Patten, which have been carried on durins: the last 

 three years through the generous support of Mr. AlUs, 

 of Milwaukee. 



So far then, as we now understand the genesis of 

 sense-organs, both in the vertebrates and in the inver- 

 tebrates, the evidence all points to the derivation of the 

 paired eyes of vertebrates from segmental sense-organs. 

 The development of the vertebrate eyes has never been 

 studied from this stand-point ; but the subject is a most 

 inviting one, and offers a broad field for observation and 

 reflection. 



The existence of an unpaired median eye in verte- 

 brates, which has been claimed by a number of recent 

 investigators, is rendered doubtful by Professor Ley- 

 dig's careful researches. If the pineal organ turn out 

 to have been a visual organ, it will present a difficulty 

 not easy to dispose of on the hypothesis of derivation 

 from segmental sense-organs. All such sense-organs 

 are paired, and a single median eye could arise from 

 them only through the fusion of at least one pair of 

 eyes. We have examples of such fusion in the inver- 

 tebrates ; but it might be extremely difficult to find any 

 evidence favoring a double origin of the pineal organ. 

 Investigation must lead with a searching analysis of 

 structure and development in every group of verte- 

 brates, while keeping up the search for a homologous 

 structure in the invertebrates. 



We have now followed the subject of the metamerism 

 of the vertebrate head- far enough to get a clear idea of 

 its essential features and general bearings. We started 



