208 



BOTANY 



PART I 



2. MORPHOLOGICAL EVIDENCE. Certain facts are inexplicable on 

 the theory of special creation, while they are naturally explained on 

 the theory of descent. The common morphological plan of construction 

 exhibited by the members of a systematic group, such as a genus, a 

 family, or a class, is of this nature. It extends in a sense to all 

 organisms as shown in the cellular structure and the nature of proto- 

 plasm. On the other hand, the theory of evolution may explain the 

 unexpected occurrence of certain features in a group when the plan 

 of construction would not have led us to anticipate them (e.g. the 

 spermatozoids in the pollen-tube of the Cycadeae). The great groups 



of the Bryophyta, Pteridophyta, and 

 Gymnosperms, with all their morpho- 

 logical differences, are essentially 

 similar in the course of development 

 and alternation of generations, and 

 in the construction of their sexual 

 organs. Only on the assumption of 

 a blood relationship can one under- 

 stand how organs of different species, 

 that appear completely different and 

 perform different functions, prove on 

 morphological investigation to be 

 homologous, or that the organs of 

 one and the same organism are so 

 frequently homologous in spite of 

 their diverse structure and functions. 

 For example, thorns and tendrils are 

 F '^:^fl^ "transformed" leaves, stipules, sterns, 



purpurea ; C, Gratiola officinalis ; D, Or TOOtS ; the Cotyledons, SCalc-leaVCS, 



Veronica Chamaedrys. The sterile stamens "bracts, sepals, petals, Stamens, and 

 are represented by black dots, and the . f ,, 



position of completely aborted stamens by Carpels Ot a plant are all 



crosses. (D after EICHLER.) formed" foliage leaves. All these 



metamorphoses of organs have evi- 

 dently taken place during the phylogenetic development. In the 

 same way reduced functionless organs found in some plants have been 

 derived from plants in which the corresponding organs are still well 

 formed. In the family of the Scrophulariaceae (Fig. 235) the number 

 of stamens ranges from five in Verlascum to two in such forms as 

 Calceolaria ; in the genus Scrophularia one stamen of the five is present 

 in a reduced condition, while this stamen is wanting in Digitalis ; in 

 Gratiola two fertile and two reduced stamens are present, in Veronica 

 two fertile stamens only, and in Calceolaria only two half-stamens. 

 Useless reduced organs are difficult to understand on the theory of 

 special creation. Occasionally an unfamiliar character appears in a 

 plant which can only be regarded as a reversion to a long-lost feature 

 of its ancestors ; examples are afforded by the occasional fertility of 



