TYPES OF VERTEBRATE AND INVERTEBRATE EYES 47 



both median and lateral eyes seem to have been evolved from the simple 

 ocellus. In other cases a particular organ having lost its original function 

 is gradually moulded or transformed so as to fulfil a different purpose. In 

 the case of the pineal organ, or epiphysis, of birds and mammals, opinion 

 is still divided upon the question as to whether this structure is being 

 transformed into a secretory gland or is being left as a derelict vestige, 

 which is gradually disappearing or has already disappeared, as seems to be 

 the case in crocodiles. 



That causes of general defects of development may be limited in their 

 action to the development of particular organs is a proposition that may 

 be answered in the affirmative both with respect to ontogenetic and 

 phylogenetic development. Thus if diminution or loss of function of an 

 organ be taken as an example, it is obvious that the full development of an 

 organ such as a muscle or sense-organ will be curtailed by want of function 

 during the life of an individual, and it may also be inferred that the poorly 

 developed muscles of domestic animals living in confined spaces, such as 

 rabbit-hutches, as compared with those of wild animals, are defects of 

 development which are inherited, because the stimulus — exercise or light 

 — which brings about the full development of the organ has been absent 

 in many successive generations. 



Also, as we shall see later, primarily paired organs such as the olfactory 

 or visual, if their function is curtailed or actually ceases, may in the course 

 of phylogeny be reduced in size, displaced, fused into a single organ, and 

 finally disappear. As examples of this we need only mention : (1) the 

 fusion which has taken place of the paired olfactory organs in the class 

 Monorhina, which includes among its living representatives the degenerate 

 hag-fishes and the lampreys ; and (2) the formation of a single median 

 eye by the fusion of paired vestiges of median eyes whose function has been 

 usurped by the gradual evolution of highly differentiated lateral eyes, as 

 has occurred in many of the arthropoda. 



The inheritance of a grave defect such as cyclopia which is commonly 

 associated with other defects, such as absence of the mouth and nasal 

 passages and defective growth of the brain, is obviously excluded, as the 

 individual is not viable ; moreover, the cause of the defect, as we have 

 seen, is in many cases due to injury, defective nutrition, poisons, or other 

 environmental conditions acting in a deleterious manner on the already 

 fertilized ovum and more particularly during the gastrular and neurular 

 stages of its development. 



