CHAPTER IV. INTRODUCTION. 123 



archicarp into a spore, sporocarp, or sporophyte is not possible without a sexual 

 act of conjugation, and hereby, to speak generally and without regard to indi- 

 vidual details, a morphological complication is introduced. The simplest case 

 of the kind, that of the most typical conjugation, may be described as the union of 

 two similar archicarps. In other cases male sexual organs are formed differing from 

 the archicarps. The production of these organs is an essential distinction ; they are 

 essential segments of the stage of the development in which the archicarp 

 is formed, and are absent from the other stage which is developed from the 

 archicarp. The two stages have therefore been naturally distinguished as 

 the sexual and the asexual; the distinction is quite correct since it corresponds 

 to that which is the almost universal rule, but it is to be observed that the 

 sexual function of the segment in question is of no moment in morphological 

 consideration and comparison; and the distinction by putting forward the sexual 

 function lays stress on that which is not generally essential, because, as we learn from 

 cases of parthenogenesis or apogamy, sexual processes may entirely fail, the 

 segments which have usually sexual functions being functionless or altogether 

 wanting, without causing any essential change in the entire course of development. 



It is indeed true that cases of parthenogenesis and apogamy do occur in which 

 the elimination of the sexual functions is accompanied by a change in the course of 

 the development ; speaking figuratively, by a partial flaw or displacement in the curve 

 which represents it. This is not always the case, as for example in Chara crinita. 

 Instances of it occur in the apogamous Ferns, when the sporophyte, instead of being 

 developed out of the archicarp, shoots out from the prothallium beside it or without 

 its formation at all. It appears still more strikingly in the formation of adventitious 

 embryos observed by Strasburger in certain Phanerogams. Here the course of 

 development shows a distinct interruption of the strict homology with the nearest 

 allied species, though the homology is at once and completely restored as the de- 

 velopment proceeds. In such cases therefore we may speak of interrupted and restored 

 homologies. 



A further complication of the course of development is introduced by the assign- 

 ment of the sexual functions and the corresponding morphological differences to 

 different individuals, each the product of a single spore ; by this means the differences 

 are carried in extreme cases as far back as to the parent-spore, or, as in the Phanero- 

 gams, to the sporogenous segments of the sporophyte. This brief notice of the above 

 well-known phenomena is all that is required in this place. 



In the cases which we have been considering the ripe spore usually separates 

 from its connection with the parent-organism and developes under favourable 

 conditions into a morphologically and physiologically independent individual, to use 

 the customary phrase, or into a bion. The sum-total of the bions produced from one 

 parent-organism (and from its sisters) is termed a new or the next youngest generation. 

 Solution of continuity and development of the separate segments into a new 

 and independent generation may also occur at other places in the course of the 

 development than that of spore-formation ; the embryo of the Ferns and Phanerogams 

 which is developed out of the archicarp is an instance of the kind. If the solution 

 of continuity occurs several times and at dissimilar places in the course of the develop- 

 ment, as for example in the Ferns, the course of development is divided into unequal 



