Si 2 THE POPULAR SCIENCE MONTHLY. 



River is a thriving community of tLis species, the first having been 

 placed there by Dr. James Lewis. 



Dr. R. E. C. Stearns,* in commenting on the occurrence of My a 

 arenaria in San Francisco Bay, states that the first record of the 

 species in California was made by Dr. Newcomb in 1874. Within a 

 few yeai-s it has increased in great numbers, furnishing a new food- 

 supply for the people. The evidence that it is a recent introduction 

 is seen in the fact that so large and conspicuous a species could not 

 have escaped the eye of the collector. No trace of it has ever been 

 found in the numerous shell-heaps of California, though it is found on 

 the Asiatic coast, from Kamtchatka to the southernmost limits of 

 Japan. Dr. Stearns believes it to have been imported w r ith the oyster 

 transplanted from the Atlantic coast. From large numbers of the 

 shells that I measured, the low index would show that it came from 

 some southern point on the Atlantic coast. 



The delicate balance of conditions between organisms, whether it 

 be between individuals of the same species or between widely-sep- 

 arated groups, is an important feature in the question of survival. 

 Professor S. A. Forbes,f in a thoughtful study of certain species of 

 entomostraca in Lake Michigan and the surrounding waters, calls 

 attention to the important part played by these minute crustaceans, 

 showing how they furnished almost the entire food for young fishes, 

 larger crustaceans and even insect larva?. He writes : " Mollusca, one 

 would say, could afford to be indifferent to them, since they neither 

 eat them nor are eaten by them, nor seem to come in contact with 

 them anywhere, through any of their habits or necessities. But for 

 this very reason these two classes afford an excellent illustration of 

 the stringent system of reactions by which an assemblage of even the 

 most diverse and seemingly independent organisms is held together. 

 ... If there were no entomostraca for young fishes to eat, there would 

 be very few fishes indeed to feed upon mollusca, and that class would 

 flourish almost without restraint ; while, on the other hand, if there 

 were no mollusca for the support of adult fishes, entomostraca would 

 be relieved from a considerable part of the drain upon their numbers, 

 and would multiply accordingly." He is much struck with the fact 

 that in the larger bodies of water, the species of entomostraca show 

 an inferior development in numbers, size, and robustness, and in repro- 

 ductive power. Their smaller number and size are doubtless due to 

 the relative scarcity of food. " The difference of reproductive energy, 

 as shown by the much smaller egg-masses borne by the lacustrine 

 species, depends upon the vastly greater destruction to which the 

 paludinal Crustacea are subjected. Many of the latter occupy waters 

 liable to be exhausted by drought, with a consequent enormous waste 

 of entomostracan life. The opportunity for reproduction is here 

 greatly limited in some situations to early spring alone and the 



* " American Naturalist," vol. xv, p. 362. f Ibid., vol. xvi, p. 537. 



