TRANSACTIONS OP THE SECTIONS. 115 



portant, from tlieir frequency of occurrence and invariability, are those that are 

 congenital in their origin, while those that are least often met with are " acquired," 

 i. e. later in their development. While the former are " general " in a physiolo- 

 gical sense, the latter are special and peculiar to the smaller groups, perhaps to 

 one family only. Further illustrations were given in explanation of these and 

 other characters, for the purpose of showing their applicability to particular cases. 

 In estimating the value to be attached to certain characters, it is necessary to con- 

 sider the purpose for which they are required. If the object be synthetical, if we 

 are seeking points of resemblance, so as to be enabled to gTOup together a large 

 number of forms into one or more large aggregates, stress must be laid, in the first 

 instance, on the congenital characters, as serving to bind together the greatest 

 numbers ; then on those dependent on frequency of occuiTence and special physio- 

 logical office, afterwards on such others as may be forthcoming. If the object be 

 analytical and discriminative, the special physiological characters demand the first 

 attention, then those which have the merit of frequency and invariability, and then 

 those that are congenital. The systematist can very rarely act up to his own 

 standard. Individual cases have to be treated on their own merits, philosophy has 

 to be sacrificed to expediency, but the tact and insight of a first-class naturalist 

 often lead him to make combinations, or to allocate forms, on what seem mere 

 grounds of expedience, but which afterwards prove, when :Oiller evidence is gained, 

 to be strictly consistent with philosophical views. 



The Eev. A. M. Normai^ made some remarks in introducing to the notice of the 

 Section the following important letter from Prof. WyviUe Thomson : — 



" Belfast, Aug. 7, 1869. 



" My deae Noeman, — You are already aware that, during the first cruise of 

 this year, Mr. Jeffreys and his party dredged and took most important thermome- 

 tricai and other observations to a Septli of 1470 fathoms. When I took Mr. Jef- 

 freys"s place for the second cruise, it was the intention to proceed northwards, and 

 to work up a part of the north-west passage, north of Rockall. I found, however, on 

 joining the ■\essel, the gear in such perfect order, all the arrangements so excellent, 

 the weather so promising, and the confidence of our excellent commander so high, 

 that, after consulting with Captain Calver, I suggested to the hydrographer that 

 we should turn southwards, and explore the very deep water off the Bay of 

 Biscay. I was anxious that, if possible, the gi-eat questions of the distribution of 

 temperature &c., and of the conditions suitable to the existence of animal life, 

 should be finally settled ; and the circumstances seemed singularly favourable. No 

 thoroughly reliable soundings have been taken beyond 2800 fathoms, and I felt 

 that if we could approach 2500, all the gi'and problems would be virtually solved, 

 and the investigation of any greater depths would be a mere matter of detail and 

 curiosity. The Hydrographer at once consented to this change of plan ; and on the 

 17th of July we left Belfast and steered round to Cork, where we coaled, and then 

 stood out towards some soundings, about a couple of hundred miles south-west of 

 Ushant, marked on the Admiralty charts 20C0 fathoms and upwards. On the 

 20th and 21st we took a few hauls' of the dredge on the slope of the great plateau, 

 in the mouth of the Channel, in depths from 75 to 725 fathoms, and on the 22nd 

 we sounded with the ' Hydra ' sounding-apparatus, the depth 2435 fathoms, with a 

 bottom of fine Atlantic chalk-mud, and a temperature registered by two standard 

 Miller-Six's thermometers of 36°-5 Fahrenheit. A heavy dredge was put over in 

 the afternoon, and slowly the gieat coils of rope melted from the ' Aunt-Sallies '— 

 as we call a long line of iron bars, with round wooden heads, on which the coils 

 are hung. In about an hour the dredge reached the bottom, upwards of three 

 miles off. The dredge remained down about three hours, the Captain moving the 

 ship slowly up to it from time to time, and anxiously watching the pulsations of 

 the accumulator, ready to meet and ease any undue strain. At nine o'clock p.m. 

 the drums of the donkey-engine began to turn, and gi'adually and steadily the 

 ' Aunt-Sallies ' filled up again, at the average rate of about 2 feet of rope per second. 

 A few minutes before one o'clock in the morning 2 cwt. of iron (the weights fixed 

 500 fathoms from the dredge) came up, and at one o'clock preciselv a cheer from 



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