4 INTRODUCTION 



of the higher forms nature may be said to have tried a variety of 

 experiments in the mode of reproduction ; on the whole there is a con- 

 tinual advance, but still by no means infrequent fallings back to simpler 

 modes ; and unless this law of retrogression is taken into account, any 

 system of classification must be pro tanto imperfect and misleading. 



If these considerations have any weight, it is not surprising that, 

 although the system of classification of Thallophytes above alluded to 

 has been adopted by a few authorities in this country and on the 

 Continent, it has not met with general acceptance. The adoption of its 

 leading principle, that ' in each class Fungi have diverged as ramifications 

 from various types of A.lgae,' ' is seen to lead to such startling results as 

 the collocation in the same class of Spirogyra and Mucor, of Volvox and 

 Peronospora, of Callithamnion and Agaricus. It may, on the contrary, 

 be safely asserted that several of the most important groups among 

 Fungi (take, for example, the Uredinese and the Basidiomycetes) display 

 no traces of genetic affinity with any known class of Algse ; and if, on the 

 other hand, we have forms like Saprolegnia and Chytridium among Fungi, 

 or Leptothrix and Beggiatoa among Protophyta, which betray strong indi- 

 cations of a degraded affinity with groups of Algse, this by no means 

 contradicts the general law that Fungi as a class form an altogether 

 independent series. 



Retrogression may take the form of the suppression of either the 

 vegetative or the reproductive organs ; and wherever you have one of 

 these sets of organs displaying strong development, while the other set 

 of organs is very feeble or altogether wanting, you have prima facie 

 evidence of retrogression. Of this examples will be given in the 

 sequel. 



While, therefore, we adopt the Protophyta as a primary class, with 

 the general limits proposed by Sachs, we have no hesitation in reverting 

 to the time-honoured division of the higher Thallophytes into the two 

 great groups of Algse and Fungi. 



The classification of Fungi adopted is that of de Bary, consisting of 

 a main series (called the series of the Ascomycetes), composed as follows : 

 (i) Peronosporeae (with Ancylistese and Monoblepharis), (2) Sapro- 

 legnieae, (3) Mucorini or Zygomycetes, (4) Entomophthoreae, (5) 

 Ascomycetes, (6) Uredines ; and of divergent groups as follows: (7) 

 Chytridinese, (8) Protomyces and Ustilagineae, (9) Doubtful Ascomycetes 

 (Saccharomyces, &c.), (10) Basidiomycetes. 



The groups 1-4 are Phycomycetes, and 7 and 8 of the second series 

 go with them ; while 9 stands in relation to 5, and 10 to 6; and they are 



' Sachs's Text-hoof:, p. 244, foot-note. 



