20 THE POPULAR SCIENCE MONTHLY. 



lution. Nay, we have but to cast our eyes over the rest of the 

 world and cyclical change presents itself on all sides. It meets 

 us in the water that flows to the sea and returns to the springs ; 

 in the heavenly bodies that wax and wane, go and return to their 

 places ; in the inexorable sequence of the ages of man's life ; in 

 that successive rise, apogee, and fall of dynasties and of states 

 which is the most prominent topic of civil history. 



As no man, fording a swift stream, can dip his foot twice into 

 the same water, so no man can with exactness affirm of anything 

 in the sensible world that it is.* As he utters the words, nay, as 

 he thinks them, the predicate ceases to be applicable ; the present 



germ (A) gives rise to tissues and organs ; while another part (B) remains in its primitive 

 condition, or is but slightly modified. The moiety A becomes the body of the adult and, 

 sooner or later, perishes, while portions of the moiety B are detached and, as offspring, con- 

 tinue the life of the species. Thus, if we trace back an organism along the direct line of 

 descent from its remotest ancestor, , as a whole, has never suffered death ; portions of it 

 only have been cast off and died in each individual offspring. 



Everybody is familiar with the way in which the " suckers " of a strawberry plant be- 

 have. A thin cylinder of living tissue keeps on growing at its free end, until it attains a 

 considerable length. At successive intervals it develops buds, which grow into strawberry 

 plants : and these become independent by the death of the parts of the sucker which con- 

 nect them. The rest of the sucker, however, may go on living and growing indefinitely, 

 and, circumstances remaining favorable, there is no obvious reason why it should ever die. 

 The living substance B in a manner answers to the sucker. If we could restore the conti- 

 nuity which was once possessed by the portions of JB, contained in all the individuals of a 

 direct line of descent, they would form a sucker, or stolon, on which these individuals would 

 be strung, and which would never have wholly died. 



A species remains unchanged so long as the potentiality of development resident in B 

 remains unaltered ; so long, e. g., as the buds of the strawberry sucker tend to become 

 typical strawberry plants. In the case of the progressive evolution of a species, the devel- 

 opmental potentiality of B becomes of a higher and higher order. In retrogressive evolu- 

 tion the contrary would be the case. The phenomena of atavism seem to show that retro- 

 gressive evolution that is, the return of a species to one or other of its earlier forms is a 

 possibility to be reckoned with. The simplification of structure which is so common in the 

 parasitic members of a group, however, does not properly come under this head. The 

 wormlike, limbless Lerncea has no resemblance to any of the stages of development of the 

 many-limbed active animals of the group to which it belongs. 



* Heracleitus says " nora/jtif ykp OVK ecr-rt Sis 3/j.&rjvai TU> oArif " ; but, to be strictly ac- 

 curate, the river remains though the water of which it is composed changes just as a man 

 retains his identity though the whole substance of his body is constantly shifting. 



This is put very well by Seneca (Ep. Iviii, 20, Ed. Ruhkopf) : " Corpora nostra rapiun- 

 tur fluminum more, quidquid vides currit cum tempore ; nihil ex his quae videmus manet. 

 Ego ipse dum loquor mutari ista, mutatus sum. Hoc est quod ait Heraclitus ' In idem flu- 

 men bis non descendimus.' Manet idem fluminis nomen, aqua transmissa est. Hoc in 

 amne manifestius est quam in homine, sed nos quoque non minus velox cursus praeter- 

 vehit." [Our bodies are carried away as a river : all that you see runs down with time : 

 nothing still remains the same : even while I say these things are changed, I am changed 

 myself. This is what Heraclitus means, when he says, " We go not twice into the same 

 river." The river still keeps its name but the water passeth away. This indeed is more 

 manifest in a river than in man ; but yet as swift a course carries us likewise away. 

 Morelfs translation.] 



