455 



SCIENTIFIC SIDE-LIGHTS 



lecules 



of the niter, producing a gas. ELISHA GRAY 

 Nature's Miracles, vol. ii, ch. 27, p. 221. 

 (F. H. & H., 1900.) 



2229. MOBILITY OF ATTENTION IN 

 CHILDHOOD Lack of Organized Activities 

 Preoccupation by Immediate Sensation. 

 Sensitiveness to immediately exciting sen- 

 sorial stimuli characterizes the attention of 

 childhood and youth. In mature age we 

 liave generally selected those stimuli which 

 are connected with one or more so-called 

 permanent interests, and our attention has 

 grown irresponsive to the rest. But child- 

 hood is characterized by great active energy, 

 and has few organized interests by which 

 to meet new impressions and decide whether 

 they are worthy of notice or not, and the 

 consequence is that extreme mobility of the 

 attention with which we are all familiar 

 in children, and which makes their first 

 lessons such rough affairs. Any strong sen- 

 sation whatever produces accommodation 

 of the organs which perceive it, and abso- 

 lute oblivion, for the time being, of the task 

 in hand. This reflex and passive character 

 of the attention, which, as a French writer 

 says, makes the child seem to belong less to 

 himself than to every object' which hap- 

 pens to catch his notice, is the first thing 

 which the teacher must overcome. It never 

 is overcome in some people, whose work, to 

 the end of life, gets done in the interstices 

 of their mind - wandering. JAMES Psy- 

 chology, vol. i, ch. 11, p. 417. (H. H. & Co., 

 1899.) 



2230. MODIFICATION OF ANIMAL 

 STRUCTURES Adaptation to New Condi- 

 tions Changes of Form in Ocean Depths. 

 The abysmal fauna is not, in fact, remark- 

 able for possessing a large number of primi- 

 tive or archaic forms. It is mainly com- 

 posed of a number of species belonging to 

 the families and genera of our shallow- 

 water fauna that have, from time to time, 

 migrated into greater depths and become 

 modified in their structure in accordance 

 with the extraordinary conditions of their 

 new habitat. There is very good reason to 

 believe that this migration has been going 

 on from time immemorial, and consequently 

 w r e find a few forms typical of the bygone 

 times, left to struggle for existence with the 

 more recent immigrants from shallow wa- 

 ters. HICKSON Fauna of the Deep Sea, ch. 

 5, p. 87. (A., 1894.) 



2231. MODIFICATION OF PARTS IN 

 ORCHIDS Evidence, of Gradual Change 

 Adaptation "by Alteration of Function. It 

 is interesting to look at one of the magnifi- 

 cent exotic species [of orchids] or, indeed, 

 at one of our humblest forms, and observe 

 how profoundly it has been modified, as com- 

 pared with all ordinary flowers, with its 

 great labellum, formed of one petal and 

 two petaloid stamens with its singular 

 pollen-masses . . . with its column 

 formed of seven cohering organs, of which 

 three alone perform their proper func- 



tion, namely, one anther and two gen- 

 erally confluent stigmas with the third stig- 

 ma modified into the rostellum and incapa- 

 ble of being fertilized and with three of 

 the anthers no longer functionally active, 

 but serving either to protect the pollen of 

 the fertile anther, or to strengthen the col- 

 umn, or existing as mere rudiments, or en- 

 tirely suppressed. What an amount of modi- 

 fication, cohesion, abortion, and change of 

 function do we here see! Yet hidden in 

 that column, with its surrounding petals 

 and sepals, we know that there are fifteen 

 groups of vessels, arranged three within 

 three, in alternate order, which probably 

 have been preserved to the present time 

 from being developed at a very early period 

 of growth, before the shape or existence 

 of any part of the flower is of importance 

 for the well-being of the plant. DARWIN 

 Fertilization of Orchids, ch. 8, p. 245. (A., 

 1898.) 



2232. MODIFICATIONS WROUGHT BY 

 MAN ON THE EARTH Limits of Human 

 Power. The modifications in the system of 

 which man is the instrument do not, perhaps, 

 constitute so great a deviation from previous 

 analogy as we usually imagine; we often, 

 for example, form an exaggerated estimate 

 of the extent of our power in extirpating 

 some of the inferior animals, and causing 

 others to multiply, a power which is cir- 

 cumscribed within certain limits, and which, 

 in all likelihood, is by no means exclusively 

 exerted by our species. The growth of hu- 

 man population cannot take place without 

 diminishing the numbers or causing the 

 entire destruction of many animals. The 

 larger beasts of prey, in particular, give 

 way before us; but other quadrupeds of 

 smaller size, and innumerable birds, insects, 

 and plants, which are inimical to our inter- 

 ests, increase in spite of us, some attacking 

 our food, others our raiment and persons, 

 and others interfering with our agricultural 

 and horticultural labors. We behold the 

 rich harvest which we have raised by the 

 sweat of our brow devoured by myriads of 

 insects, and are often as incapable of ar- 

 resting their depredations as of staying the 

 shock of an earthquake or the course of a 

 stream of lava. LYELL Principles of Ge- 

 ology, bk. i, ch. 9, p. 150. 



2233. MOLECULES OF GASES Dis- 

 tance and Size Infinitely Small Molecular 

 Theory Infinite Minuteness Overwhelms the 

 Mind. The very remarkable properties of 

 gases, their apparently unlimited elasticity 

 and indefinite powers of expansion, were 

 very difficult to explain on any theory of 

 their molecules being subject to such at- 

 tractive and repulsive forces as seem to ex- 

 ist in other states of matter. A considera- 

 tion of these properties, together with the 

 power of diffusion by which gases of very 

 different densities form a perfect mixture 

 when in contact, and the fact that by the 

 application of heat almost all liquids and 



