No. 546] SHORTER ARTICLES AND DISCUSSION 373 



sustentative selection to distinguish it from that which is so de- 



Sustentative selection is generally thought to be nearly always 

 in operation. In every group of animals in which T have made 

 any special field observations, namely, bryozoa. birds and 

 beetles, the falsity of this belief has been impressed upon me. 

 With fresh-water bryozoa the food supply can scarcely ever be 

 taxed. The limiting conditions seem to be largely inconstancy 

 of the bodies of water, the danger of being eaten, and the limited 

 extent of suitable substrata. 



With birds, when one really sees an emaciated individual, the 

 result of some accident which has made it difficult for it to ob- 

 tain food, one is struck by the very great contrast with other 

 birds. In my experience, in skinning birds in the state of Wash- 

 ington, summer and winter, I never opened one not well equipped 

 with abdominal fat, On the other hand, the great loss of the 

 young birds by adverse weather and the large variety of pre- 

 daceous enemies is common knowledge. 



With lady beetles there is a more direct relation of the num- 

 bers to food supply, but even here it is a question of finding food, 

 rather than any real lack of it. The food supply of the adult 

 beetles, embracing aphids, pollen and spores, is superabundant. 

 The principal causes of death seem to me to be due to inability 

 of the females to distribute the eggs proportionately to the dis- 

 tribution of aphids, and, secondly, the unreliableness of aphid 

 stocks, owing to their rapid annihilation when one of the numer- 

 ous aphid diseases or parasites becomes rampant. One may 

 often hunt over many colonies of aphids without finding any 

 coccinellid larva?, and then at last find one of the same species 

 with several large egg masses. So many larva* will hatch in this 

 case that they will consume all the aphids before they themselves 

 have all become mature. As a result, they will wander, only a 

 few surviving who may have the efficient instincts and good for- 

 tune necessary to discover another stock of aphids. 



Of the half dozen or so species of the large coccinellids mast 

 common in the United States, it is common to find here one 

 species abundant, and there another, though some others are also 

 found. Attempts to account for these contrasts in numbers on 

 grounds of temperature, humidity, altitude and the like, have 

 proved unsuccessful. I think it is because the instincts of egg- 

 laying and of migration of the larvae of one species or another is 

 better adapted to the species of aphids and aphid-affected plants 



