THE SUCCESSION OF ANIMAL FORMS 171 



If we were to follow the guidance of those curious analogies 

 which present themselves when we consider the growth of the 

 individual plant or animal from the spore or the ovum, and the 

 development of vegetable and animal life in geological time — 

 analogies which, however, it must be borne in mind can have 

 no scientific value whatever, inasmuch as that sim.ilarity cf 

 conditions which alone can give force to reasoning from an- 

 alogy in matters of science, is wholly wanting — we should ex- 

 pect to find in the oldest rocks embryonic forms alone, but of 

 course embryonic forms suited to exist and reproduce them- 

 selves independently.^ 



I need not say to palaeontologists that this is not what we 

 actually find in the primordial rocks. I need but to remind 

 them of the early and remarkable development of such forms 

 as the Trilobites, the Lingulidae and the Pteropods, all of them 

 highly complex and specialized types, and remote from the 

 embryonic stages of the groups to which they severally belong. 

 In the case of the Trilobites, one need merely consider the 

 beautiful symmetry of their parts, both transversely and longi- 

 tudinally, their division into distinct regions, the necessary com- 

 plexity of their muscular and nervous systems, their highly 

 complex visual organs, the superficial ornamentation and micro- 

 scopic structure of their crusts, their advanced position among 

 Crustaceans, indicated by their strong affinity with the Arach- 

 nidans or spiders and scorpions. (See figures prefixed.) 



* I may be pardoned for taking an example of the confusion of thought 

 which this mode of reasoning has introduced into Biology from a clever 

 article in the Contemporary written by a very able and much-esteemed 

 biologist. He says : " The morphological distance between a newly hatched 

 frog's tadpole and the adult frog is almost as great as that between the 

 adult lancelet and the newly hatched larvae of the lamprey." The "mor- 

 phological distance" truly, but what of the physiological distance between 

 the young and adult of the same animal and two adult animals between 

 which is placed the great gulf of specific and generic diversity which with- 

 in human experience neither has been known to pass ? 



