344 



SCIENCE. 



[Vol. XXI. No. 542 



bers, and cause widespread consternation. No doubt climatic 

 and other influences have much to do with these sudden up-ris- 

 ings, as we find species that are known to be parasitized but 

 very little, which fluctuate in numbers greatly with different 

 seasons. All the effect, then, cannot be attributed to parasites. 



The difference between the work of parasites and other influ- 

 ences, is quite marked and distinct in certain channels, and can 

 be easily traced. The tendency of parasites is to increase or de- 

 crease in numbers as the host is numerous or scarce. A few 

 years ago the wheat aphis was so numerous over the wheat plants 

 that it threatened to destroy the whole crop in this region. Pres- 

 ently certain of the aphis looked brown and swelled, which told 

 plainly that the parasites were there too. The wheat grew and 

 headed. Still the aphis increased by the thousands daily, and the 

 parasites increased in numbers also. Then there came a time 

 when the parasites were in the majority, and, before the wheat- 

 beads had ripened, a live aphis was a scarce and hard thing to 

 find. The next year the wheat aphis was not common, and what 

 did appear were disposed of early by the parasites. 



Sometimes the work of the parasites is not as prompt as the in- 

 stance just cited. For illustration, the oak army worm, Edema 

 albifroiis, was never known to be numerous enough to greatly 

 injure the oak till two years ago, when the species came in such 

 nuaibers as to strip whole forests of their foliage. Of the sever- 

 al hundred caterpillars and pupse collected, only one pupa was 

 parasitized. Last year the trees were again stripped by count- 

 less numbers as the year before, but from the pupse collected, 

 about every one in ten was parasitized. Probably this year the 

 caterpillars will be less numerous, and by next will be scarse, 

 because of the work of parasites. 



An ideal parasite is one that would keep its host in such 

 camplete subjection that no outbreak would occur, and the num- 

 bers not great enough to do any harm. While the effect of par- 

 asitism is not ideal in every respect, it nevertheless is a boob to 

 economic entomology, and has already been used to good advan- 

 tage, by introducing many foreign parasites that are known to 

 work on certain species. As we become still more familiar with 

 these parasites and their hosts, much more good, through parasit- 

 ic species will undoubtedly result. 



EARTHQUAKES IN AUSTRALASIA.— II. 



BT GEORGE HOGBBN, M.A., SECRETARY OF THE SEISMOLOGICAL COM- 

 MITTEE OF THE AUSTRALASIAN ASSOCIATION FOR THE ADVANCE- 

 MENT OP SCIENCE. 



In my former communication I explained the nature of the 

 work that the Seismological Committee of the A. A. A. S. pro- 

 poses to do anent earthquakes. In the present contribution I 

 shall endeavor to sketch briefly what has already been done for 

 New Zealand earthquakes. 



The committee has published two reports, 1891 and 1892. 

 The former was drawn up by Sir James Hector, F.R.S., and 

 deals with New Zealand earthquakes to the end of the year 1890. 

 It contains a list of earthquakes (537) felt in New Zealand from 

 the earliest settled times, and gives interesting details concern- 

 ing the somewhat severe shocks of October, 1848, and January, 

 1855. The last-named is notable as being one of the few in any 

 country in which movement of the land has been actually ob- 

 served by skilled observers on the spot. Captain Drury, R.N., 

 was engaged at the time on the nautical survey of the New Zea- 

 land coast, and, being in the neighborhood of the land raised, 

 was able by actual re-measurement to confirm the general im- 

 pression. " An area of 4,600 miles was estimated to have been 

 raised from one foot to nine feet, the greatest elevation being on 

 the west side of the Wairarapa Valley, the vicinity of Porroria 

 Harbor not being affected, and the west side of Cloudy Bay, 

 north of Blenheim, having actually been depressed to the extent 

 of five feet." (Transactions, A. A. A. S., 1891, p. 533). The 

 elevation has been permanent. The same report (1891) contained 

 a map by Sir James Hector, showing the seismic areas, principal 

 fault and earthquake-rents in the islands of New Zealand. 

 The division into seismic areas is not, however, based upon the 



determination of the earthquake origins, and, in the opinion of 

 the present writer, is on that account misleading. At the same 

 meeting of the Australasian Association (Jan., 1891), I reada pa- 

 per on New Zealand Earthquakes, which contained a somewhat 

 larger list (775) than the report of the committee, together with 

 two maps and a diagram showing curves of monthly seismic fre- 

 quency, the New Zealand curve based upon the records of 745 

 shocks being compared with Mallet's curves for the Northern and 

 Southern Hemispheres — 5,879 and 333 earthquakes, respective- 

 ly — (See Milne on Earthquakes, p. 356). The record for New 

 Zealand shows a maximum of frequency in September, with 

 smaller maxima in January and March, and minima in April 

 and October, November, December. The inclusion of these facts 

 might modify Mallet's curve for the Southern Hemisphere, but 

 it does not appear that they point to any connection between 

 earthquake-frequency and the season of the year. 



One of the maps exhibited showed, by shading, the earthquake- 

 frequency of the shocks in various parts of New Zealand, the re- 

 gion most effected being a portion of Cook Strait, included in the 

 triangle Wellington, Blenheim, Wanganui; the next shade of 

 frequency includes Christchurch, the nest. Nelson. There is an 

 isolated district of local earthquakes round Rotorua and Tara- 

 wera. On my other map were marked the epicentra of 35 earth- 

 quakes for which the data were sufficient to ascertain them with 

 any degree of probability, and I have since been able to deter- 

 mine more or less exactly the origins of a few of the earthquakes 

 of 1891-1893. The two chief sources are situated — (1)10 miles 

 north of Lake Siimner, or about 80 miles north-northwest of 

 Christchurch. Hence proceeded the shock which on the 1st of 

 September, 1888, threw down the upper ijortion of the spire of 

 Christchurch cathedral. To the same origin I am able definitely 

 to assign 10 other shocks, and probably many more belong to it. 

 (3) 45-50 miles north-northwest of Wellington, in Cook Strait. 

 This and some other origins near it are accountable for most of 

 the New Zealand shocks, the average intensity being very low, 

 III. — IV., on the Rossi-Forel scale. 



The method used for finding the origins has been, in general, 

 founded on the observed times of the shock at the several places 

 at which it was felt, with the help of the isoseiemals, when the 

 effects were sufficiently definite to assign the degree of intensity 

 on the Rossi-Forel scale. 



One somewhat striking point in connection with all the recent 

 earthquakes in New Zealand, is the low velocity of propagation 

 they possess (less, with one exception, than 30 miles a minute). 

 At first this made me doubt the correctness of the calculations, 

 but the large number of shocks for which the velocity can now 

 be approximately ascertained renders the results tolerably cer- 

 tain. In the solitary exception (an earthquake of the present 

 year, which I am still investigating) the velocity is probably be- 

 tween 45 and 55 miles per minute. The depth of the origin has 

 not been found in many cases, but in those for which the solu- 

 tion of the equations is most satisfactory, the depth is in each 

 case about 24 or 35 miles below the surface. 



LETTERS TO THE EDITOR. 



#*# Correspondents are requested to be as brief as possible. The vtnter'8 name 

 is in all cases required as proof of good faith. 



On request in advance, one hundred copies of the number containing his 

 communication will be furnished free to any correspondent. 



The editor will be glad to publish any queries consonant with the character 

 of the journal. 



Some Recently Discovered Trilobites with Appendages. 



The past winter the Geological Department of Columbia Col- 

 lege came into possession of some extremely interesting specimens 

 of Triarthrus Beckii, which were discovered by Mr. W. S. 

 Valiant, now of Rutgers College, in the Utica shales at Rome, 

 N.Y. They were entrusted to W. D. Matthew, our fellow in 

 geology, for complete description, and Mr. Matthew's paper, re- 

 cently read before the New York Academy of Sciences, will ap- 

 pear in the Transactions of the Academy for May. Owing to the 

 unavoidable delay in their issue, and because the subject is such 

 an interesting one, this preliminary announcement is made. The 

 Trilobites possess two undoubted antennse that come out together 



