CLARK ON THE BREEDING HABITS OF HOLOTHURIANS. 83 



THE CLASSIFICATION OF BIKDS. 



HUBERT LYMAN CLARK. 

 (Abstract. Published in full in the Auk, October, liM)l.) 



The tendency of recent systems of avian classification is toward a great 

 increase in the number of orders. Such orders are not equivalent to the 

 orders of other zoological groups. So far as possible orders should be, 

 and usually ar(\, based on the most important character of the class. 

 At any rate, the character or characters taken for a basis should be 

 those least likely to vary with habits. Among birds such a character 

 is to be found in the arranfjemcnt of the pierylw. There are apparently, 

 judging from such knowledge as we now have, ten types of pterylosis. 

 Each one of these might therefore be taken as the basis for an order. 

 Such a plan is not free from difficulties, but tliere is certainly much 

 to be said in its favor. 



THE BREEDING HABITS OF HOLOTHURIANS. 



HUBERT LYMAN CLARK. 



The following brief sketch of the breeding habits of holothurians is 

 not intended as a complete account, but is merely to call attention to 

 some facts of general interest in connection with the way in which those 

 animals care for their young. Like most marine animals, the Echino- 

 derms as a rule, set their eggs free in the Avater and there they are 

 fertilized and there the young develop. Such a method reminds one 

 forcibly of those i>lants which rely on the wind to scatter the pollen; 

 and carry it to the ovules, which without it must die. In the one case, 

 it is necessary to produce incalculable millions of spermatozoa and 

 countless ova. in the other equally incalculable numbers of pollen-grains. 

 Both cases illustrate what we term extravagant or wasteful methods. 

 As we study the line of development among both plants and animals, 

 it is interesting to note how this wasteful method of reproduction be- 

 lomes modified in the higher forms of life, until at last very few young 

 are produced at a time and those are carefully tended. Now the holoth- 

 urians are without doubt the most specialized form of echinoderms and 

 it is instinctive to see how, as their peculiar type of structure has 

 developed, there have appeared a number of instances of improved 

 methods of reproduction. There can be no doubt that the struggle for 

 existence among the minute, free-swimming inhabitants of the ocean 

 is verv severe, and verv likelv is more severe with the introduction of 



