SPOiXGES. 59 



well known that there is no ascertained limit to the action of heredity, and that not 

 only observable characteristics, but even habits and tendencies may be directly trans- 

 mitted from one generation to another. There is a universal and necessary law of 

 heredity which can be used in bridging the gap between the Protozoa and the Metazoa 

 which may be briefly formulated as follows : All animals exhibit a tendency to inherit 

 the characteristics of their ancestors at earlier stages than those in which these charac- 

 teristics first apj)eared. Thus, if an ancestor or radical form acquired a new character 

 or took on a new habit when adult, this would tend to reappear in the descendants at 

 earlier and earlier stages, and in course of time would become carried back to the 

 adolescent, then to the larval stages, and finally either become useless and disappear 

 altogether, or if useful, and therefore retained, become restricted to embryonic stages. 

 This law was first advanced in 1866, by three jiersons, Haeckel, €<)))(■, and the writer, 

 almost simultaneously. 



The Ascones are certainly, so far as known, the simplest or most generalized of the 

 Metazoa, and approximate to the Protozoa in such a way that it is possible with the 

 aid of the law of concentration of development to explain the transformations by which 

 such an organism could have risen from the Protozoa. The egg of all Metazoa is in its 

 first stages a simple cell, and like all other cells is a homologue of an individual or 

 zoon among the Protozoa. This primitive egg cell has but one mode of growth by 

 which it forms tissues. It divides or segments and builds up the i^riraitive tissues of 

 the embryo by a similar process to that by which colonies are formed among the Pro- 

 tozoa. Tlierefore the egg after segmentation is no longer a single zoon, equivalent to 

 a single zoon among the Protozoa, but a mass of such zoons, differing from the mass 

 of a free colony of amoeboid Protozoa in about the same way that the included cells of 

 an ampulla differ from a colony of Flagellata. 



Among colonial Metazoa we frequently find on the same adult stock, individuals 

 <levoted to the performance of distinct functions, and having their shape and structure 

 so modified thereby as to differ wi<lely from each other, though in their younger stages 

 they were more nearly alike. 



Thus, on the same hydrozoan stock we may find females loaded with eggs, males 

 carrying only sperm cells, sexless polyps devoted wholly to alimentary purposes, others 

 with only defensive functions. "We have therefore excellent reasons for assuming that 

 similar transformations took place in the transition from the simple colonies of the 

 Flagellata to the more complex condition of the sponges, and we can make a jiicture 

 of these changes in strict accord with the laws of morphology. 



Throughout tlie Metazoa as well as in sponges, the external layer or ectoderm is 

 protective and builds the protective armor, scales, etc. ; the mesoderm is essentially 

 devoted to the formation of flesh and organs of su]iport, while the endodei-m is devoted 

 to the function of digestion, and the elaboration of all the parts concerned in this pro- 

 cess; but everywhere these three layers are derived from one cell (the egg). If now 

 we imagine a series of changes, beginning with any flagellate protozoon, and follow- 

 ing out the indications of embryology, we should first have a sheet of attached flag- 

 ellate feeding forms ; secondly, these surmounting or arched above a base composed 

 .solely of supporting indivi<luals without collars or flagella ; thirdly, the outermost 

 losing their flagella and collars, would become simply protective pavement cells, 

 while the central ones retain their digestive functions; the change slowly becoming 

 more complete, and the central ones acquiring a capability of being withdrawn into 

 the interior when alarmed. The last step would be the inheritance of the invaginated 



