Some advocates of animal welfare have pointed out the ethical issues associated with spreading Earth-based wild-animal life by terraforming. In particular, they claim it may be ethically objectionable to bring into existence large numbers of animals that suffer greatly during their often-short lives in the wild. There are also concerns that even with full terraformation, distinct differences between Earth and Mars, such as gravity, lengths of the day and night cycles, and differing/lacking magnetic fields, would cause harm to many introduced species that have evolved for millions of years under Earth conditions. Though some species may survive, and others possibly could be adapted through genetic modification, if the introduced species were isolated on Mars and not frequently interbred with Earth counterparts, the species would eventually evolve through many generations in order to better suit their new environment, possibly leading to different evolutionary lines. Thus, the introduced life may eventually look and act very different from their Earthly counterparts and/or ancestors.