. The theory of evolution in the light of facts. its growth. The Water Ferns are small plants with a hori-zontally growing stem which creeps on the ground orfloats on the water. They are heterosporous— they produce in separate vesselsmicro- and macrospores ; thetrue Ferns are homosporous,with only one kind of spore. Class 2: Equisetinse(Horsetails). (a) Herbaceous (all thepresent forms) ; the leavesrelatively small, undivided,arranged in whorls, andbranched (Fig. 30). (b) The spore capsules aresituated on peculiarly meta-morphosed leaves which are conjoined into a single bloom (so called)
. The theory of evolution in the light of facts. its growth. The Water Ferns are small plants with a hori-zontally growing stem which creeps on the ground orfloats on the water. They are heterosporous— they produce in separate vesselsmicro- and macrospores ; thetrue Ferns are homosporous,with only one kind of spore. Class 2: Equisetinse(Horsetails). (a) Herbaceous (all thepresent forms) ; the leavesrelatively small, undivided,arranged in whorls, andbranched (Fig. 30). (b) The spore capsules aresituated on peculiarly meta-morphosed leaves which are conjoined into a single bloom (so called) which terminatesthe growth of the shoot. The branches are arrangedin whorls— they spring at even distances and all inthe same plane on the stem. Class 3 : Lycopodinae (Club-mosses). (a) Herbaceous (all the present ones); the leavesvery small and simply constructed (almost scale-like). (h) The sporangia (spore vessels) are situated singlyon the base of the upper side of the leaf, or in the leafaxis, or above the axis on the stem Fig. 30 .—Habit of an Equise-TUM {Equisetum arvense). a,barren shoot; b, fertile shoot;c, portion of stem. DOMAIN OF EVOLUTIONARY HYPOTHESIS 129 The fertile leaves, which are united in specialclusters at the tips of the shoots (stalks) and henceterminate the growth (Fig. 31), are very often differentlyformed to the barren ones. (2) General results of systematic classification corn-fared with those of falceontology. From the above it is clearthat a priori we cannot knowwhether our systematic cate-gories are really primary anddo not signify forms of animalsand plants which are reducibleone from another, truetypes in the sense of elements which presentthemselves in the definition ofa type —plan of construction(symmetry, habit as a whole),and the degree of difierentiationin tissues and organs for thegeneral vital activities—can inthemselves be subject to variation. It may thereforehappen that family and class character
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