Review. 
t la¬ 
the detailed statement of the phenomena involved which is 
presented in the present work. 
The method adopted is to begin with a “ Statement of the 
Working Hypothesis” (Part I.) which occupies no less than 254 
pages; then to give a “ Detailed Statement of Facts ” of the 
development, structure and relationships of the different types of 
Archegoniate sporophyte (Part II., 402 pages); and finally to 
present a summarised “ Conclusion ” (Part III., 109 pages). 
After a short introduction, setting forth the general theory, in 
which all botanists will agree, that the main progress of evolution 
within the plant-kingdom is a gradual and progressive adaptation to 
terrestrial life, Professor Bower contributes a thoughtful chapter on 
the “ Scope and Limitations of Comparative Morphology,” and then 
proceeds, by gradual steps, to come to close quarters with his 
“ working hypothesis.” After setting out the leading facts of the 
alternation of generations as seen in the life-history of a fern and 
discussing the “ balance ” of the two generations in the different 
groups of Archegoniatae, he deals in two useful chapters with the 
cytological distinction between the generations (including a discussion 
of the extremely important recent work of Farmer and his 
collaborators on the cytology of apospory and apogamy) and with 
the varied and complicated phenomena of alternation seen in the 
Thallophyta. The author’s treatment of this last subject is parti¬ 
cularly able and acute. 
Professor Bower next proceeds to discuss “ the biological 
aspects of alternation,” restating the theory embodied in his paper 
already referred to. Admirably worked out as that theory has been 
by the author, it is, however, not quite legitimate to identify the 
“ biological ” theory of alternation with the “ antithetic ” theory, as 
is apparently done on p. 86. No one doubts that the factors of 
increased spore-production, of the appearance of typical land-roots, 
of a differentiated conducting system, and of the general elaboration 
of the sporophyte, have been the important factors in adapting the 
sporophytes of vascular plants to a terrestrial existence. What is 
doubted is the evolution of this sporophyte from a body of spores 
produced by the segmentation of the zygote ; and it is quite possible 
to derive the terrestrial sporophyte, with its gradually increasing 
adaptation to drier conditions, from a spore-bearing thallus. The 
“ biological theory of alternation,” so far as it concerns the Pteri- 
dophytes themselves, does not, then, necessarily rest upon an 
“ antithetic ” basis. 
In his next chapter Professor Bower deals with “Sterilisation,” 
i.e. the conversion, in the course of descent, of sporogenous into 
vegetative tissue. The actual existence of this process is amply 
demonstrated in a varied series of instances, and the contention of 
the author “ that plants show not uncommonly to-day such a 
conversion of cells from the propagative to the vegetative state as 
the antithetic theory would demand” is fully borne out. In his 
three chapters dealing with sporangia, the author presents most 
valuable analyses of the nature of these organs and their relations 
to the vegetative development of the plant as a whole. 
With the eleventh chapter, “ The Theory of the Strobilus,” we 
come to the central point of Professor Bower’s hypothesis, and it 
is here, naturally enough, that those who have come to other 
conclusions as to the probable ancestral history of the Pteridophytic 
