504 SCIENCE PROGRESS. 



a similar effect on animals most remote from one another 

 in structure and systematic position. Complete or nearly- 

 complete, and constant darkness is one of these conditions. 

 Two modifications which are generally observed in animals 

 living in darkness are diminution in the pigmentation of the 

 skin and deterioration of the eyes. In most cases there is 

 abundant evidence that the animals passing their lives in 

 darkness have but recently diverged from forms which live 

 in daylight. We may inquire then to what extent the 

 ancestral condition is recapitulated in the ontogeny of these 

 forms, with regard to the two adaptations mentioned. Not 

 long ago this question was raised in a controversy concern- 

 ing the possibility of explaining the loss of the eyes in cave- 

 inhabiting animals by the selection of individual variations. 

 Professor Lankester had suggested that the individuals 

 which had the best eyes were better able to find their way 

 out of the dark recesses into which they were accidentally 

 carried, because they would detect a glimmer of light which 

 those whose eyes were less perfect would not notice, and the 

 former moving towards the source of the light would emerge 

 into the open air again. It seemed to me that this hypothesis 

 was contradicted by the fact of the recapitulation of the 

 eyes, because when very young all the individuals are able 

 to see, and would therefore all be able to escape in the 

 manner suggested. 



But we must turn to the results of direct observation in 

 order to ascertain to what extent this is correct, whether the 

 eyes are fully developed in the immature stages of cave 

 animals which are blind when adult. Unfortunately obser- 

 vations on the ontogeny of such animals are by no means 

 complete or detailed, but still enough has been ascertained 

 to show that recapitulation is neither perfect nor altogether 

 wanting. 



Troglocaris Schmidtii is a crustacean of the caves of 

 Europe, which is quite blind in the adult condition ; it was 

 found by Dr. Joseph that the embryo while still within the 

 egg was provided with eyes. 



In the adult Proteus of the caves of Adelsberg in Dal- 

 matia, the eyes are very small and entirely covered by the 



