Final answer:
The similarities between the organ systems of mammals and fish are best described by 'Descent with modification' which means they have evolved from a common ancestor but modified these structures to adapt to different environments. Structures can be homologous due to common ancestry or analogous due to convergent evolution. Therefore correct option is C
Step-by-step explanation:
The best description that explains the similarities between the organ systems of mammals and fish, considering they are both vertebrates but have very different evolutionary paths, is C) Descent with modification. This principle, put forth by Charles Darwin, suggests that both mammals and fish inherited their structures from a common ancestor and have modified these structures over time to suit their different environments. This evolutionary concept opposes spontaneous generation, and it denotes the gradual changes in populations of organisms over generations.
Homologous structures in organisms, such as the limbs of mammals and the fins of fish, are due to their common ancestry, which diverged into various life forms adapting to different environments. On the other hand, analogous structures, like the fins of fish and the flippers of dolphins, arise not from direct common descent but rather due to convergent evolution, where different species have independently developed similar features as adaptations to similar environmental challenges.