August 26, 1892.] 



SCIENCE. 



123 



by natural forces" shall depend "only upon the changes in the 

 positions of the particles, and not upos the paths by which, or 

 the velocities with which, they have moved from the old positions 

 to the new." 



Now let P denote the magnitude of the stress between any 

 two particles of a system and r the distance between those parti- 

 cles; then Pdr is the work done by this stress during an infinitesi- 

 mal displacement of the system. The work done by the stresses 

 between all particles of the system during a finite displacement 

 is 2 J Pdr, in which the summation is extended to all pairs of 

 particles and the integration covers the whole displacement of the 

 system. Now if '2 Pdr is the differential of a function of the 

 quantities r, the value of the integral will depend only upon 

 the initial and final relative positions of the particles. But the 

 assumption that each P is a function of the corresponding r only 

 (in accordance with the proposed foxirth law) is only one of many 

 irossible assumptions, any one of which would make 2 Pdr the 

 differential of a function of the quantities r. The mathematical 

 statement of the condition that 2 Pdr shall be a perfect differential 

 is given in treatises on " Differential Equations." 



It thus appears that the principle of the conservation of energy 

 does not require the truth of the proposed fourth law. The law 

 may be true nevertheless; but it may well be questioned whether 

 its truth is established with any such degree of probability as 

 would entitle it to rank with the laws of Newton as a fundamental 

 hypothesis of dynamics. L. M. Hoskins. 



Madison, Wis., Aug. 16. 



The Black-Knot. 



On p. 10, Vol. XX.. of Science appears an instructive chapter on 

 "Black-knot," a fungous disease of the plum and cherry, of 

 much trouble to cultivators. A point of additional scientific in- 

 terest is that this fungus illustrates a principle long since pre- 

 sented by the writer of this, that nature does not place species 

 where it is for the best interests of the individuals of the species, 

 but generally has some ulterior purpose not always apparent to 

 us who are eager to uncover her intentions. For instance, there 

 are numberless trees and shrubs that struggle along in swamps, 

 and are rarely found elsewhere, and these have come to be known 

 as "swamp-lovers," but close observation has shown that the 

 same tree or shrub will thrive immeasurably better when removed 

 to dry ground. 



On the writer's grounds is a specimen of Clethra ulnifolia, fif- 

 teen feet high and as much wide on an especially dry spot, and 

 growing with a luxuriance rarely seen inthesv^ampy spots where 

 nature has located the plants. Some reason has been found for 

 the appearance of these plants in swamps and not in dry ground 

 naturally in the fact that the seeds will not sprout in dry, but 

 only in wet ones. It looks like a. fiat of nature. " Though you 

 would like to grow in dry places you shall not Something must 

 grow for my purposes, in swamps, and you have to do it." They 

 can only be found where the seed will sprout. 



It has always seemed to the writer that it was one of the weak- 

 nesses of many discussions in the study of development, that it 

 was generally from the individual standpoint. Nature cares only 

 for the individual, therefoi'e questions of nutrition, fertilization, 

 and others are all viewed in their relation to the plant's " strug- 

 gle for life." It seems rather that nature cares but little for the 

 individual, and stands ever ready to sacrifice the whole stock 

 when it interferes with some purpose, which we have seldom 

 been able to fathom. 



Coming to the black-knot on the plum and cherry, we have 

 here a destructive American species Sphm-ia (Plowrightia) mor- 

 bosa, of little injury in its native state, thriving amazingly when 

 it can get as a host-plant the European domestic plum or Euro- 

 pean morello cherry. It thrives in these cases with a vigor it 

 never shows at home. I have seen it in many parts of the east 

 on the wild dwarf choke-cherry, Cerasus Virginiana; in Colorado 

 on its close ally Cerasus demissa; in North Carolina sparingly on 

 Primus chicasa; and in the White Mountains on the red cherry, 

 Cerasus Pennsylvanica. Recently in driving through various lo- 

 calities on Mt. Desert Island, it was seen on the latter much 



more abundantly than in any of the former cases noted ; but 

 never anywhere with the amazing destructiveness it presents in 

 these garden representatives of foreign species. In Pennsylvania, 

 and probably other States, the cultivated cherry has been wild 

 for over a hundred years. It is abundant, and in some cases so 

 nuDierous as to be the chief element in a piece of woodland. 

 But though it is evidently the foster-child of the cherry and not 

 the plum, it prefers the plum and the sour cherry. The knot is 

 rarely found on the wild cherry trees of the sweet cherry species. 

 For all its long hereditary cherry taste, it rushes to the plum 

 and the morello with as much avidity as if long-continued "en- 

 vironment " had induced the love. 



It seems to be forgotten in many discussions of the black-knot 

 that it is an American parasite, and that it may be found in 

 quantities everywhere that the botanists look for it. When, 

 therefore, the State of New York tries to " stamp it out" by leg- 

 islating against garden trees affected with the fungus, it seems 

 like bailing out the ocean with a bucket. Of course, cutting 

 down and burning destroy many spores, but the wild nests send 

 forth myriads of young to take the places of the domesticated foes 

 destroyed. Thomas Meehan. 



Germantown, Pa. 



Hectoring a Hawk. 



Early one morning in August, while concealed in the grass and 

 bushes of a White Mountain meadow, I saw an interesting en- 

 counter between a sharp-shinned hawk and a number of blue-jays 

 and pigeon woodpeckers. Four of the woodpeckers were quietly 

 preaning themselves in a dead pine by the lake shore, when sud- 

 denly a small and beautifully proportioned hawk dashed into their 

 midst. They scattered shrieking, and found shelter in a fringe of 

 woods near by. Their cries brought a kingbird to the spot, and 

 the hawk was promptly attacked by the pugnacious fly-catcher 

 and compelled to follow the flickers into concealment. The king- 

 bird, satisfied with routing the hawk, hovered away over the 

 meadow out of sight, and not long after the hawk reappeared and 

 perched in the dead tree. 



From time to time one or more of the woodpeckers came back 

 to the tree and were at once charged by the hawk. In each in- 

 stance they showed superior speed and escaped by their rapid 

 flight. Their noise attracted the attention of a flock of about 

 twenty blue-jays, and presently the blue-winged pirates came 

 sailing Qver the meadow by twos and threes. As they neared the 

 dead pine the hawk darted downward after their leader. The jay 

 plunged quickly into the bushes, uttering wild cries and squawks, 

 which were re echoed by his companions. The hawk returned 

 to the pine squealing pettishly, and the jays closed in upon him. 

 They scaled the lower branches of the dead tree; they capped the 

 neighboring maple saplings and alders ; they watched for chances 

 to brush past the hawk on his perch, and they assailed him with 

 all the invective of their ample vocabulary. They threw them- 

 selves into the sport, as they seemed to regard it, with all the 

 energy of boys playing "short fox." 



The hawk took the matter much more in earnest ; for he was 

 hungry, and striving for a breakfast. Again and again he shot 

 from the lofty branches of the pine, aiming first at one jay, then 

 at another. By and by all the flickers returned, and added to 

 the confusion by their cries and rapid excursions around the tree. 

 The hawk in several instances seemed to lack but a single wing- 

 beat of success, but the hour drew on without his making a 

 capture. He grew weary. His plumage showed the chafing of 

 the bushes. He chose lower and lower branches for his rests, and 

 finally his sallies seemed directed more towards clearing the tree 

 of noisy birds than to the capture of any one of them. At last he 

 abandoned the dead pine and perched in trees having foliage. The 

 jays followed him jeering, and he shifted his ground slowly until 

 he gained the woods and disappeared. Then the jays crowded 

 into the lower branches of the pine, hopped up from limb to limb 

 until one after another gained the summit, and proved to the 

 whole meadow that they had won the battle and fairly worried the 

 hawk away. 



The drama seemed to me to be significant in two ways ; first, as 



