 In contrast, a species can only exist at time \(t_n\) if either it or a parent species existed at \(t_{n-1}\) and there was some relationship of spatial contiguity between component individuals of the species at \(t_n\) and the individuals belonging to either the same species or the parent species at \(t_{n-1}\). This is because of the essential role of the causal relationship of heredity. Heredity generates both the coherence across a population requisite for the existence of a species and the variability of predominant traits within the population, without which a species would not evolve.