78 



DISCOVERY 



ami while liis conclusicnis will tlmil>tli-ss iiut-t witli some 

 adverse critiiisni, they represent a solid contribution 

 which is in general harmony with the ideas of certain other 

 investigators who are dissatisfied with the more orthodox 

 views on evolution. 



Dr. Arber's essay was written after the publication of 

 the first of the series of memoirs by Dr. Kidston and 

 Professor Lang on the structure of the Middle (or possibly 

 Lower ?) Devonian plants discovered a few years ago by 

 Dr. Mackie in Aberdeenshire ; but he did not live to see 

 the later parts, the contents of which would probably 

 have led him to reconsider some of his conclusions. Dr. 

 Church's essay on " The Thalassiophyta and the Subacrial 

 Tran.smigration," referred to in a previous number of 

 Discovery, would undoubtedly have appealed with 

 considerable force to Dr. Arber had he lived to see it. 



Unfortunately, most of the remains of the oldest known 

 land-plants preserved in the rocks of the earth's crust are 

 fragmentary, and with few exceptions afford little evidence 

 of anatomical characters; the literature in which they 

 are described is scattered, and in many instances difficult 

 to obtain. But such records as there are demand the 

 closest scrutiny by seekers after origins. Dr. Arber's 

 concise descriptions of the several genera are particularly 

 welcome, aided as they are by many well selected and 

 admirably reproduced illustrations. The older Devonian 

 flora, called by the author the Psilophyton flora, from 

 the occurrence in many localities of the genus Psilophyton, 

 differs widely from the latter, or Archaopleris, flora, 

 which includes undoubted ferns and other Vascular 

 Cryptogams, with a few representatives of trees presumably 

 bearing seeds. It is contended that the members of the 

 Psilophyton flora should be regarded as Thallophyta, 

 despite the fact that in their organisation they had 

 advanced beyond the limits of that group as we now 

 understand it. The most highly differentiated living 

 thallophytes are certain seaweeds adapted to life in water, 

 but in the older Devonian thallophytes it is maintained 

 that we have an added complexity of structure which 

 was the result of their transference to a terrestrial habitat. 

 The different groups of viiscular plants represented in the 

 Upper Devonian rocks are believed to have been derived 

 from different ancestors which were algal in nature, a 

 view which has much in its favour. 



In Dr. Arber's book the student conversant with the 

 general trend of modern speculation on the evolution of 

 plants and with recent palaeobotanical researclics will 

 find much to interest him and to make him think. 



A. C. Seward. 



Instinct and the Unconscious. By W. H. R. Rivers, 

 M.D., F.R.S. (Cambridge University Press, 

 1920, i6s.) 



Let it be said at the outset that Dr. Rivers has given us a 

 book that must be read and discussed, and that will be 

 enjoyed whether or no it will be agreed with. Although 

 confining himself to broad principle?, he yet breaks new- 

 ground ; and by laying stress on the biological side of 

 psychology, he does a service to both sciences. 



The contrast between a treatise such as this and the 

 textbooks of psychology in vogue twenty and thirty 

 years ago is remarkable. There is a new spirit in the air, 

 and the new spirit is the outcome of a new method. In 

 chemical science, the outstanding feature of modem times 

 has been the rise of physical chemistry. The study of 

 processes has proved the most illuminating and economical 

 method of dealing with the multiplicity of fact ; the 

 dynamic has supplanted the static. 



The same change is coming over psychology. We used 

 to think of a curious being called the Ego installed some- 

 where in the chariot of the brain, and driving a rather 

 mixed team of horses — Will, Intellect, Emotion, Memory. 

 There was, of course, a certain amount of interaction — 

 now and again Emotion would kick Intellect, Memory 

 would jib and pull the rest up, or Will would need a touch 

 of Ego's whip ; but on the whole there wasn't much 

 trouble of that sort, and good driving would get the team 

 along. 



To-day, charioteer, chariot, and horses alike are lost, 

 merged in the conception of a single co-ordinated whole 

 whose parts, though differentiated, are all in a state of 

 equilibrium one with another, so that one cannot be 

 altered without altering all the rest. 



The most important single aspect of this state of 

 balance is the dominance of some parts over others. The 

 whole nervous system, whether regarded on its mental 

 or its purely neural aspect, is a hierarchy, in which every 

 part is being controlled from above, and is controlling 

 other parts below. Normal mental health consists in 

 striking the right balance between the lower centres— 

 for the most part those concerned with the primitive 

 instincts — and the higher centres — those concerned with 

 altruism, with the highest types of emotion, and with 

 the reason. As Dr. Rivers points out, this balance can 

 be upset in either of tw'o opposite ways, but with the 

 same general result. Either the subordinate processes may 

 be unduly strengthened, or the controlling processes may 

 weaken. The conflicts of adolescence are due to the 

 former cause, the neurasthenia and so-called shell-shock 

 of adults to the latter. 



It is of the utmost interest to find close parallels between 

 the mode of interaction of the parts of the mind and that 

 (at least in the simpler, more plastic organisms) of the 

 parts of the body. 



In the organism, regarded merely as a physiological 

 machine, we are also brought face to face with a system 

 composed of a number of parts in equilibrium, some parts 

 being dominant over others. It is impossible to go into 

 details here, but those who care to pursue the subject 

 in books like Professor Child's /orfifirfKa/i/vi" Organisms'^ 

 will find the most suggestive parallels. 



How comes it that such parallels exist ? It proceeds 

 from the fact tliat the only type of living system which 

 can succeed in the struggle for existence must possess 

 both diversity and unity. It must possess diversity to 

 cope with the different problems propounded to it by 

 the environment, it must have unity in order to sub- 

 ordinate one function to another according to circum- 



' C. M. Child. Individuality in Organisms. Chicago, 1915. 



