200 LECTURES ON BIOLOGY 



With a machine-like precision and inevitable necessity one 

 phase follows another until the * inherited form has become 

 complete.' 



If there were no progress on earth, if the conditions remained 

 always alike, this statement would be correct, but under present 

 circumstances it is impossible to agree with it in this rigid form. 

 The organism is more than a machine, for it is able to adapt 

 itself. The course of development, too, is, to a certain degree, 

 subject to external influences : certain stages may, under certain 

 circumstances, be suppressed, not only for a short time, but also 

 permanently ; even the succession of developmental stages may 

 be altered. A few observations will prove that this statement is 

 incontrovertible. 



We distinguish among the mosses two generations : an asexual, 

 which reproduces by means of unicellular spores ; and a sexual, 

 which originates from the spores and produces male and female 

 germs. From the fertilized egg-cell develops then once more the 

 asexual generation which is represented by the stalked capsules. 

 These two generations alternate in a regular cycle. In the 

 ferns we observe a little deviation from this scheme, in that there 

 proceeds from the spore at first the so-called protonema from 

 which the moss-plant proper grows with its sex-organs. But, as 

 Klebs has shown in the minute Funaria hygrometrica, it is 

 possible by withdrawing the light to suppress the sexual genera- 

 tion. While normally the life of the protonema was but brief, 

 and was extinguished soon after the development of the moss- 

 plant, it grew in the dark almost continuously for two years with- 

 out producing germ-cells. The early form remained therefore 

 alive far beyond its ' destined span.' 



Like the mosses, the ferns have an alternation of generations. 

 Normally the spore is followed by the prothalltis which carries 

 the sex-organs, then impregnation takes place, and from the 

 impregnated egg develops the asexual generation, the fern 

 proper, which later proceeds to spore-formation. This succession 

 is as definite and regular as the course of development in the 

 higher animals and in man. But under special conditions we 

 may succeed in shortening the development, and in omitting not 

 only the most comprehensive and important stage, the fern- 

 plant itself, but in suppressing at the same time the development 



