JANI'ARY 25, 1895.] 



SCIENCE. 



95 



senc-o of true flowers until late geologic 

 times, for it is only by the visits of insects 

 and their irritating action on vegetal proto- 

 plasm that the most irregular flowei-s have 

 heiMi slowly evolved, for there is a broad 

 parallelism l)etween the more dillerentiated 

 types of the vegetal kingdom and the ap- 

 pearance of the various orders of insects, 

 which was : 



GEOLOGICAL SUCCESSION OF IXSECTS. 



Devonian, Ortlioptera (eai^mgs, grasshoppers), Xeu- 



roptera ( ant-lions) . 

 Carlwniferous, Coleoptera (beetles). 

 Cretaceous Olite, Hymenoptera (bees), Hemiptera 



(lice), Diptera (flics). 

 Tertiary. Lepidoptera i Imtterflies). 



We know from the close association of 

 insects and flowers that the insects were 

 modifled by their visits to flowers, and con- 

 vei-scly that flowers have been changed to 

 suit the visits of insects, and it is therefore 

 not improbable that our most highly spe- 

 cialized flowers, and most irregular ones, 

 appeared and were modified by the Lepidop- 

 tera in the late Tertiary time ; for moths 

 and butterflies are most higlily specialized 

 to insure cross fertilization, or allogamy. 

 This variation in flowering plants must have 

 been most strong at the close of the Mio- 

 cene period, and after the retreat of the 

 glaciei-s still more rapid than before, for it 

 is proljable that the intense struggle which 

 took place by the migration and intermix- 

 ture of forms of difterent kinds, occasioned 

 by the change of environmental conditions, 

 was a powerful factor in causing the strik- 

 ing variety of flowers and insects. The 

 ' responsive power ' of the proto{)lasm of 

 the plants, acting in concert with the exter- 

 nal impulses received from the environ- 

 ment, must have been strong after the dis- 

 appearance of the glaciers, on account of the 

 re-occupation of a barren glacial country 

 by northward moving plants, whose proto- 

 I)lasm had become re.sjwnsively mobile dur- 

 ing the long continued struggle in the south. 

 It is not at all improbable that the poly- 



petalous groups of plants were northern 

 ones during the Miocene period, and that 

 their flowering period depends on this past 

 geographical position. Those plants which 

 lived far north during late ^Miocene and 

 Pliocene times were least modified, for if is 

 likely that moths and butterflies were then 

 few in number, and the time was not suffl- 

 ciently long for change to take place before 

 the glacial ice sheet moved southward, mix- 

 ing the northern and southern types, and 

 introducing a struggle which was to last 

 until the ice disapi)eared by the temperate 

 heat. Many tropical plants remained asso- 

 ciated with the northern forms crowded 

 southward by the glaciers, notwithstanding 

 that a great number perished under the 

 more rigorous conditions of a colder climate. 

 When the glaciers retreated, the predom- 

 inant polypetahe adapted to a cold climate 

 did one of three things : 1 , They reti-eated 

 northward. 2. They retreated up the high 

 mountains. 3. They took almost exclusive 

 possession in their growth of the spring 

 months, for the temperature conditions are 

 such as to suit well their hereditarilj- im- 

 pressed preference for the cold. 



These plants flower and mature their seeds 

 quickly before the summer is well advanced, 

 which mark them as physiologically adapted 

 to the influences of the short glacial summer, 

 alternating with the long glacial winter. 

 This rapid growth production of flowers and 

 seed in a short space of time is possible from 

 the quantity of nutritive material stored up 

 in the plant. The beet, turnip, i)arsnip and 

 carrot are familiar examples of biennials 

 with the reserve substance packed in the 

 roots ; the houseleek, lily and onion with 

 the ba.ses of the leaves enlarged and thick- 

 ened to contain the stores of starch, sugar 

 and [jroteids. Even under these favorable 

 conditions, when tlie plant would be in a 

 condition to grow most vigorously, every 

 externally perceptible vital motion never- 

 theless ceases, and it is onlv after a dormant 



